If you don't know the service history, I'd replace all the fluids (brake, clutch, fork oil, engine oil, coolant). You may also want to check the valve clearances.
I haven't heard of anyone installing Ohlins

on a Tbird, but I'd be interested in any information you find. Everyone in the forum seems to swear by the Hagon. Most purchase is from
www.davequinnmotorcycles.com
The K&N isn't much of a performance boost on it's own, since the airflow is restricted by undersized inlet on the airbox anyway. It is however, a cheaper alternative to the OEM filter, since Triumph requires you to purchase the entire airbox just to replace the filter.
Other than that and the obvious items (tire tread, chain/sprocket wear, etc), you should be good to go for many miles.