Howdy Lee,
Can we assume you're in the UK? I'm in the US ... so riding requirements/opportunities may be a bit different. What "big bike" did you sell off?

It sounds like you're looking more for a road bike that can do some "light" off road? If that is the case, the DRZ400S is a pretty good pick. No idea what TRF is (some UK forum I suspect?)

I owned the "E" model DRZ (off road only). No longer made. It was the BEST of the DRZ line. There was also a K model early on. The S model is what you now get in UK, and far as I know, it has not changed a bit since it's 2002 introduction.

I would not worry about a 5 year old bike. Take your time, be willing to pay a bit more ... and find a true Virgin.

There are a few other choices but most are more OFF ROAD oriented, and most are more expensive than a DRZ400S ... and NONE are as reliable as the Suzuki:

Husky TE250, TE450, TE650 Terra
KTM 250EXC, 450EXC, 690EXC
CCM 450 (new, not released yet)
Yamaha WR450F (off road only but can be made street legal)
BMW 450 (no longer made but may be around used)
Beta 450 (excellent bike! Not sure if sold in UK)

About 250's
The current crop of 250's are pretty interesting and getting better all the time. The new (made in Thailand) Honda CRF250L is rated very high, but for most the WR250R rates highest and does most things better. It's expensive and I'm not even sure you can get it in the UK. It's street legal (unlike the WR450F I listed above)

Thing about 250's is ... they pretty much suck on fast roads and long-ish runs. If you lived or traveled in S. America or Asia ... I'd say go 250. Reason? Towns are slow and crowded, even on highways speeds fairly moderate. 250's work there.

In UK, EU or USA ... not so much. Most 250's top out at about 70 mph ... and may not be that smooth or comfortable at that speed. Off road of course they have a BIG advantage over heavier bikes if set up right. (a big IF)

The DRZ-S can be a good choice and work on road with some good mods. Find Adam Lewis's thread (Jedi Master), he rides (or rode?) a DRZ400S.

I'd say it for sure will need a custom seat. For ALL ROAD, better lights (HID),
next would be suspension work to suit and then some elegant solutions for carrying luggage. Go higher gearing for road use ... and lower for off road. Could just work.
Good luck!