Greenhouse for a newbie :) (it's me)

I’m not going to be much help to you. I’m more of a polytunnel girl. But what I can tell you is the little ones with plastic covers aren’t nicknamed blow aways for nothing    . Also you will very quickly realise that it’s not big enough for you veggie growing obsession that’s just ignited. But to be fair what ever size you go with you will wish you’d gone for the next size up. It’s a thing all gardeners suffer from   . 
So best advice I can give you is go for the biggest one you have the space for. I will let someone with more knowledge on the subject to advise on everything else.