Things do go quiet on the UK section occaisonally, although this is the quietest I've seen it for a while...!
Anyway, at the risk of stating the obvious: as you say, your options are auto or manual for the standard car - trim-wise, they are the equivalent of the US 'Limited' model (although a few so-called 'Base' models have appeared in rhd - cloth seats, no front fogs etc) - and the auto-only SRT6 - which I would recommend 'cos it don't need modding (unless you can't help yourself) - it's a real beast as built! - and it's a lot rarer - only just over 100 came to the UK, compared to approx 3000 standard cars.
Unusually, a few SRTs have been up for sale recently - expect to pay around £8k for a decent low-mileage example, although if you're lucky, you might get away with nearer £6.5k.
A good standard car will be about £4-6k depending on condition & mileage, although an earlier one with low miles & good history could be a better bet than a later on with higher miles which is only more expensive 'cos of its age - but then the paintwork might be a little better...
Talking of which, with 2003/4 cars, watch for rust around the rear wheel arches - they must be kept clean - they're not old enough for it to be bad yet (usually) but if it's there, obviously better if caught early - even some otherwise fastidious owners can neglect this vital area - and check the bottoms of the doors behind the weather seal - this can stay damp and some have been known with the beginnings of rust here; although not a problem on most cars, it's worth watching for. Finally on rust, most cars have a little around the boot latch but again, not a big deal and easily fixed if necessary.
You're probably aware that mechanically they are a Mercedes 320 SLK so are very well built and reliable - the SRT6 is an SLK32 AMG - better brakes, transmission, suspension, engine and supercharger- well worth the extra IMHO.
Some parts are getting a little harder to find but source them from Mercedes rather than Chrysler - they'll usually be cheaper! Same for servicing - an independent MB shop will usually be a better bet than a Chrysler 'stealership'!
I've done no modding myself so can't really comment on that and others will tell you about any technical issues (not that many TBH) but, if they don't, just search this forum - there's masses of stuff on here for you to do your own research.
Good luck and keep us posted!