Dave, tbh, its not so much the machine, as the products you use to cklean the upholstery. Any extraction machine will do, probably best is the vax.
The problem with using it a a shampooer is i feel it leaves the seats/carpets too wet, an they take ages to dry. I would get some meguiars apc, diluted 1:10, spray it on and work mit in with a soft nailbrush or similar, then use the vax to sook up the foam, that way your not 'overwetting' the fabric.
Hope this helps mate
Iain
As for pricing your looking at around 90-120 quid for a decent machine iirc.