Is this purchased or rented? That decides how permanent a solution you want. Your profile doesn't say where you are but at 5 acres I guess it is in the country. I am a gardener with 1½ acres rather than an allotment holder and am out in the country. The first thing to decide is how much to cultivate eventually and are you thinking just vegetables or fruit (soft fruit and top fruit) as well, then make a rough plan. Next fence off all the production area with something rabbit proof or maybe like me deer proof as well. If you can start by mowing the whole area, that way you can keep it under control and gradually change grass into beds and paths. If the soil and drainage is good I wouldn't bother with raised beds but rather deeply dug beds, easier to work on with a wheelbarrow. When I did mine I couldn't decide whether to do narrow beds or not, in the end I did 10' wide thinking I could divide to 4' later but I never have - I've found they are a good hoeing width and quite easy to look after - I have 7 beds 25' long and a couple of other patches besides and a 10'x15' Comfrey bed in the vegetable area, a 25' square fruit cage and an orchard in another part of the garden. You need space for composting, probably a shed and greenhouse and maybe cold frames. I initially divided mine up with 2' to 3' grass paths but found them a pain when they were wet, high rainfall here, and the edges take a bit of maintaining. I have eventually over the years taken the extreme solution of concreting them - that's why it is important to know if you own or rent, I think concrete is a better use of capital than raised bed sleepers. You can use membrane and slabs for some flexibility. Grassland does take a while to clear of bugs but the only way is to suffer the original damage, the first crop of potatoes is a bit sacrificial but you should get a reasonable crop and things will get better. If you keep records for your rotation and double dig a portion each year you'll gradually build up the fertility. Now there's an extreme solution for you - hope it hasn't put you off!