Hi Magic & welcome to the forum,
I like Ian am around 6'0" & probably a bit more than 100Kg or 15 1/2 Stone in old money which I understand. I fit with no problems but do not have a internal fuel tank for that reason, again as Ian says Danny muddybuggies site owner is 6'4" & fits nicely. I'll be honest I have no idea how he does but I've seen it so I know it's true.
Best bet would be to come along to one of the shows, you don't say in your profile where you are from but we may have somebody close who you could go see to have a sit in.
As to the cost of a donor, that is a very difficult question, I paid £235 but recovered £150 with bits i sold off it. There was a perfect donor the other day on another forum that was only £100 (wasn't for sale for very long) but that price is rare. Alex (Collie) bought one the other week & paid £300 I think.
As to whether you need a donor, the topic has been talked about quite a few times, try a few searches, different people have different ideas. I personally don't like Q plates, much prefer an age related one, no rational reason just don't like them, sometimes they can cause problems with insurers.
Alan...