The T300 is basically a TST with a slightly higher MTOW (bit like the difference between a 150 and 152). Given the chap is throwing in a trailer as well, if it's in good nick, that's quite cheap. You will also need to spend a bit of cash (£300-£500) on a couple of flying suits, helmets, headsets and an intercom to be able to operate the beast 2-up.
Incidentally, Older Thrusters come with one of two engines, the 503 and 532. The 503 is air-cooled and much easier to look after, but the 532 is liquid cooled, so takes a bit more maintenance work - but does give about 20% more power, which is well worth having.
By the way, there's a Spectrum out there for sale for similar money
here also; given a choice I think the Spectrum is the nicer to fly (but that's just preference, I've flown both and have nothing against either).
G