Hi Daz
welcome to the forum.
The choice of GS or GSA is up to you but if buying used, hold out for the higher TE spec bikes if you can. The main advantage I found with the GSA was the larger fuel tank range, a slightly smoother ride and less twitchy at speed in the twisties than the standard bike which I found to be a little flighty when pushed (at least compared with the GSA which just feels a little more planted due to the geometry differences). It has a little more presence on the road too if that matters for you.
Don't be put off by the growing list of "possible" faults as many of us never see many of them.
A well maintained bike might show a few issues common to the GS such as the info/trip switch sticking (test that as a replacement switch isn't cheap. My 16 plate GSA had this issue but a few squirts with contact cleaner sorted it). On the whole, they're a reliable workhorse.
Look for well maintained bikes which have been treated to regular coats of ACF to keep the paint issues and corrosion at bay. Mine still looks showroom at 16K miles so some of these reported issues have to be down to lack of regular care as well as the odd genuine issue. Frames can corrode, and this tends to show on the subframe especially if an issue, as that is vulnerable to salt and spray from winter roads. I know of one RS bike whose frame was badly corroded in under 12 months from new that the bike frame was replaced under warranty. That's a rare occurrence though.
Other marques are not immune either. I've seen as many examples of corroded engine casings on Triumphs and some jap bikes used year round.
Check that the headstock bushing has been replaced under the recall...that's a must, and also that the bike's had regular oil changes/servicing.
Also check the condition of the rear driveshaft seals. If they look ok, chances are that the driveshaft will be fine.
The wheels are worth checking for loose or missing spokes if you look at one of the spoked ones. My GSA had a spoke missing from the rear wheel. That was replaced out of warranty for free. Good dealerships do look after their customers!
Rear pad wear can be high (5 or 6K mile intervals) but it was much the same for my Triumph Tiger and trident bikes, and also for my Aprilia RSVR. My discs are fine at 16K miles, no bad wear showing at all.
Suspension can be iffy on some ESA equipped bikes with reports of failing suspension quite widespread. If it's going to happen it seems to happen within the first few years but used units can be had cheaply enough or replacement with alternatives is possible. Again, this is not unique to BMW.
On the whole the bikes seem solid and reliable, and certainly provide one of the best all round riding packages going. Like most other bikes, as long as you realise that to keep a bike in good running order it will require far more than an annual service at the dealership. Part of ownership is regular care and attention to try and ensure a preventative maintenance approach.