Hi Dan....first of all you want a two wheel one that will take a blade of around 104- 108 inches long.
If you go on ebay and have a look at Elektra Bekum saws....you couldn't get better second hand.....german build, can buy spares easily on the net ( not that I have needed more than a single capacitor in the 19 years I have owned mine, and they give a fantastic straight cut, plus they have a mouth of around 10 inches for a deep cut and a "gate"...of about 12 inches from the blade to the upright..............
I cannot say in simple words how good mine has been....chopped everything from 0.8mm ply to 8" thick tree logs, it is so good......and a blade will cost about 6 quid inc vat.
neil.