Topsoil with good compost ought to be enough, the horse manure will be best if it sits and ages for at least 12 months, best mix it with the compost and let stand. The one drawback I find with raised beds is the watering, they do dry out quickly. I mix well rotted manure with my compost and then add mushroom compost, it helps but with the dry weather we are having I am watering nearly every day.

I'm growing strawberries (3 mature plants + 4 yearlings) and courgettes (5 plants) in my raised bed. They are doing well and have provided fruit and veg for 3 of us over the last 4-6 weeks but do take up quite a bit of space. I've also got herbs in another bed. By the way slugs will still find their way into the bed!! I've got runner beans and french beans growing in tractor tyres ( a form of raised bed I suppose). Give things a try and enjoy your harvest.