If I was buying again, I'd seriously consider a Thermal Arc Fabricator 181

It has the added bonus of being a MIG ( albeit with no trolley ), ARC and TIG welder all in one. It also can go down to very low currents for fine work.
It can take a 5kg roll which is plenty really.

I wanted a trolley based machine that could take a 15kg roll when I bought mine ( so it could carry the bottle ), but in reality a 15kg roll is bloody huge.

And you can buy trolleys to mount the smaller units and carry the gas bottle anyway.

http://www.migtigarc.co.uk/MIG_Welde...ator_181i.html