According to the service schedule, the timing belt should be changed every 4 years or 100,000kms. So if the 2011 model that you're looking at has been serviced correctly, then it should've been done last year.
As mentioned above, the drivetrains between the models are the same. A tuned 250 will essentially make the same power as a tuned 275. The only differences come in the specifications. I'd recommend finding one with recaro's and xenon headlights. I'd probably also steer clear of the LE model as it doesn't come with a mechanical LSD.
Don't expect huge power gains from F4RT engine. With the mods you mentioned above, you would be lucky to see 325hp. A stock 250 with some sticky tyres will still embarrass considerably more powerful cars at the track