Hello,
Not a fussy wish list at all,
Ive had 4 greenhouse's in my time so far and the thinking your doing ref no plastic, etc is good,

Now have a look at what ive learnt and practiced over the years,
(1) A good sized door & doorway is a must "not only for getting a wheelbarrow through but even carrying bags of compost etc, if the door is to narrow you'll soon find out and you'll be the one living with it.

(2) the frame work, go for the frame work that has cross member sections both across the roof area and the wall area, these are a strength & wind idea, so ref snow and the weight they work & strong winds again they work,
The inside of the roof area should have at the appex strong secure brackets
"these give strenth to both sides of the roof and are handy for hanging baskets,
"i also store some canes in this area, and my light fittings are off these brackets,

(3) You want windows that open when the greenhouse gets to a high temp, I have 4 x window's three of these have automatic opening devices fitted, (about £25 each)

these will open when your not around and will save you there cost many times in lost plants due to over heating,

the fourth window opening i have as a opening operated by hand.

(4) Staging if your going to sow your own seeds etc also a section for potting on inside the greenhouse i find very handy,

Outside staging or a place for putting your plants for hardening off is very handy, And storing pots under

(5) the floor, it depends on what your going to have here ie concrete or earth,

Ive concrete because it keeps the heat in better and it can be washed & disinfected when needed, and the staging & weight of trays etc stand level & strong on concrete.

earth, you can have beds dug or walls for beds made,
ive found that in the deps of winter the earths damp and cold seep's into the greenhouse and this is'nt what i want for over wintering shrubs etc,
But its a choice for the user to make?
Troughs can be used for growing toms/cucuumber etc.

Under my staging on top of the concrete i have brocken roof clay tiles, these help keep the heat temp, up when im using the heating.
(6) Make sure you seal the base of the greenhouse,
you'll be amazed we're draft can get through and reduce heat in the winter,
Also that the door fits tight,
(7) Electric supply,
Its well worth having electric in the greenhouse and fitted correctly,
this not only gives you light but connection for plugs and i wouldnt be without my heated seed tray area, all this works on low heat for under the seed trays,

also heatiing with eletric heating is one way you can heat.
(8) Heat or cold greenhouse,
If your wanting to heat the average DIY greenhouse owner has electric or parrafin,
both are "now" costly, electric is handy with a timer but if bad weather causes cuts in the electric you'll need a standby and paraffin is offten used,
The modern parraffin heater are not as smelly as the older ones,

Greenhouse liners are a good way to bring down your heating cost "bubble wrap in sheet form attach to the greenhouse walls inside,
Also the use of fleece over plants in the very bad weather will help.

(9) Outside water,
the use of water collection units ie the tubs etc mounted at the corners of the greenhouse will collect your rain water for you and is handy for the greenhouse use.
(10)
one last tip,
site your greenhouse so it's handy for getting all the way around the outside area for both cleaning the glass & also if you want to use greenhouse paint on shading,
and the right site for sun.

I have a wind reflection small hedge in the direction that the winds arrive in the winter to deflect the wind off the greenhouse front,
so if you know your going to get a windy spot ?
This tip works for me.

Ive said it before and i'll say it again, You get what you pay for and when it comes to greenhouse buying!!!

Buy right and buy once buy cheap and live to regret it.

A wooden framed greenhouse is warmer inside than a metal frame but wooden framed cost more and need to be maintained we're as alloy framed dont.

If you have the space and are really a keen gardener?
Then a greenhouse to my way of thinking is a must and over the years will return its buying cost mant times over.

The greenhouse i have now is 12feet long x 8 feet wide
I bought it second hand (infact i bought two)
and its been up here in france for the last 6 yrs with no trouble.
So see what going in your area second hand.

You say first greenhouse as if you mean you'll get another one later. If that's the case, and if space allows, I'd say go for the biggest greenhouse possible now rather than later as you'll regret getting a smaller one.
Quality greenhouses will withstand the weather. I wouldn't say mine has half the features Dungy suggests but has lasted many many years.
You don't need automatic vents. Just do what most do and open them every day in the summer and close at night by hand.
Electricity is a must.
Staging is also important. Decent staging can make an 8 foot by 8 foot greenhouse equivalent to one twice the size.

Guys, remember that Sarah is asking about a very small greenhouse, only four foot deep! This is possible and you can get greenhouses that are 4' deep by 6' wide. These are very basic and with a roof vent and the door open will get all the ventilation possible.
The straightforward B&Q type house will fit your requirements, Sarah, but I would suggest that you try for a larger one to start with; you won't regret it.

I have a very basic, plastic greenhouse. I bought it off Amazon warehouse for £35 inc delivery. I originally used it to keep plants dry, but it has been invaluable in potting on seedlings and kick starting plants such as Canna and Dahlia. Mine is 4ft by 6ft.

It offers little protection from hard frosts and quickly builds up humidity. The only ventilation is rolling open the door. It survived the gales of winter with zero damage and I would recommend. It is relatively ugly but functional. I would recommend a solid floor and not earth as the above has said. It has plenty of space and an entry level greenhouse:

http://www.amazon.co.uk/Apollo-Gardening-Ltd-75162-Greenhouse/dp/B004CY7256/ref=sr_1_1?m=A2OAJ7377F756P&s=merchant&ie=UTF8&qid=1369825258&sr=1-1&keywords=greenhouse

My plan is to replace it with a wooden greenhouse this year as I want it to offer more winter frost protection and to be more long term. If you can afford £37 then go for it.

Thanks so much for all the advice

I think I have a power supply near enough that it useable - but I hadn't though about the base and getting all the way around the greenhouse for cleaning.

Will go do some more research..at this rate I might end up with a little greenhouse at the back and a bigger one at the side, but it gets so windy here that I am worried about the greenhouse being damaged.

Will let you all know the outcome, think my next steps should be to go see what pictures of greenhouses are on GoY to help me decide.

Thanks everybody for your help!