Hi Sarah,
Get the biggest you can afford cos as sure as eggs is eggs it won't be big enough in a year or two
Polytunnels are cheap (realative to greenhouses) to buy, the covers will last anywhere between 4 & 7 years before they need replacing and compared to a greenhouse the one drawback is ventelation but that being said they are extremeley useful - thats why I'm getting one for the lottie
Cloches & Frames are similar.
Cloches are usually like a mini polytunnel and are used for protecting seed/seedlings from the worsed of the weather at the begining & end of the growing season ( now & Autumn) they can be any length you like if you make thhem out of polythene but they aren't so good as a polytunnel for overwintering things in due to the small size. Frames are usually more permanent structures and are used for hardening off plants ( sort of in between area from Polytunnel to outside) although you can overwinter in frames. as they tend to be larger. Traditionally they were brick with Glass lights on and in the old walled kitchen gardens they were heated by either hot water pipes of a hot bed of manure & this was used to force early crops (carrots, beetroot, cauli's or Strawberries etc).
Ideally you want all 3 ( 4 if you can include a greenhouse) as they do have their own uses.
My coldframes are temporary, they are made sectional and fix together with 4 bolts. 1 is 7ft x 4ft and the other will end up as 12ft x 4ft (I'm about to remake it to fit on my bed at home) and they are used to take the over flow from the greenhouse arounfd this time of year although I've cut down this year so haven't needed them ...... yet