I haven't really kept an close eye on the 991 market so can't comment, the sports car market generally is in decline and people bought high and want the investment back which is why the prices are still high on the market platforms like Autotrader. It's a buyers market and would say a deal is there to be made, interest rates don't help and that is largely where the deal will be done. Doing a quick motorway or WBAC check can give you an indication on what the bottom end of the trade market is but I wouldn't use that as gospel.
Cars that have sat for a while I wouldn't be overly concerned but mainly the price will be too high and it's probably a post covid car that is in negative equity. Like I say there is always a deal, but do your research. It might be a number of factors like the paint isnt great or a slightly unfavourable spec. They are sensitive to spec and people are very picky. Like mentioned previously spec like chrono and PSE if it doesnt have it cars aren't as appealing. Same with the type of seats, the spec, 4, 14 and 18 way seats. etc etc Bose, Burmeister, colour is another big factor. Guards red is super unpopular but they seem to go on the higher end of the market same with speed yellow. Arctic silver for example seems most common and will be cheaper, GT silver on the other hand is a really sought after colour...
Everything is an option so obvious stuff is sometimes not there like dimming rear view mirrors. Purists hate the rear window windscreen wiper, I personally like it. People will literally have them removed

funny bunch
Main thing is figuring out what you want and then finding one. It can be made super complicated main thing is you like it but residuals are worth considering. Took 2 years to find our 991 and it pretty much has everything even leather air vents and seat belt covers silly stuff that we weren't looking for but it was a tick box for the previous owner which benefits you and resale.
Other age old techniques like going in end of the month and wanting a deal to increase numbers, take the finance to get a better deal and pay it back within the window to reduce the interest premium.
In terms of the window, what I meant by that is everything has been done for the year and milage. No services due for 2 years, new tyres, disks and pads done. Essentially no outstanding elements that add up quickly. The 991 is around its 12 year service window which is a big one, PDK service etc the above could be a 2-5k outlay if you don't buy well which then taints the experience and costs more money. Anything that's outstanding can be used as wiggle room.
All of those are decent outlays when you purchase. I wouldn't underestimate the tyre wear they go through them quicker and they aren't cheap needing to be N rated.
Best thing you can do is get an independent to do an PPI inspection it will cost £250-350 but you will then find out everything about the car and is super common in the Porsche world and all specialist offer it. They will give a breakdown of what's needed, when and how much. Great when buying from a private buyer or even from a dealer.
The thing about the 991 is it's the last NA so it will always be worth something to someone and the thing is cars aren't investments in fact the opposite. PVRs slightly different but even so porsche didnt fill the allocation of 50s so deals were to be had then too. Now it is seen as the ultimate 991 outside of a GT car and has the 50 moniker people really appreciate them and the prices are reflective. They really are stunning, wide body etc :wub:
Biggest regret is we were offered a 997 sport classic when they came out again as they didnt fill the allocation and they were marmite and didnt buy for those reasons, now they are worth 250k+. Nobody has a crystal ball as long as you can afford the depreciation, like any car they depreciate and cost money but they are on the better end of the scale especially now as they are 10-12 years old. Because they are older unexpected bits need to be considered but like MR.W said its all really well know and really easy to predict and sort. There is very little terminal issues and you have to be really unlucky to have something major like and engine go unlike the 996 and 7.
Depends on what your plans are, people who move into them end up sticking for a long time because there isnt much to move to without moving into really exotic territory and they really do offer so much. The other option is another 911 but the newer versions like the 992 have changed they are bigger, heavier, more powerful and less connected like all newer cars and there is a purist following of the older versions. As many purists there are as many who don't care and just want a brand new one. The Porsche brand love it or hate it has the cultist following.