Horrible, horrible newbie question

Right, brace yourself.

I'm looking for a seriously budget mountain bike for commuting and
hopefully venturing further afield in the future. When I say budget, I
mean sub 200 quid, which I realise is phenomenally tight.

I was looking at a couple of Raleigh (Motomax) and Diamonback (FS20)
models - can anyone offer any advice at all?

Also, I'm about 6ft4, is a 19" frame really going to be big enough? All
of the bikes in my price range seem to be 19" frames at the most.

I have read the FAQ and couldn't see anything stating that I shouldn't
post this, so go easy on us!

Thanks