Johns on the money.
The only problem with an oil analysis is it only gives you a small window of the cars condition. The oil is only meant to be in for 12000 or 2 years so its best to do it at the end of that stage but will only give you an idea of the last 2 years. It wont show the wear directly over the cars life. It will give a good idea there are certain metals, off the top of my head I think high copper shows they bearing material has warn through.
Depends who you ask / how the car has been used / tracked and how many owners etc There isn't a definitive answer.
The main thing you can do to ensure that the worst case isnt met is interim oil changes. I plan to have mine done at schedule and every year between for £100 in parts and an hour of your time swapping the oil out is a good idea i think.
Some bearings have gone at 30k some are over 150k and still running although I would be very worried at that point. I would say somewhere in-between isnt unreasonable. Between 80-100k.
With the age of these cars a few other big jobs can be done at the same time, gearbox/engine mounts as these are rubber they perish and one of the mounts is behind the heat shield which tends to go first. There is also the engine bolts which can shear or work their way out, at worst when you rev the engine it will lurch in the bay. There have been numerous members with this issue and for piece of mind its worth having those done at the same time.
Mine is a July or August build 2006 car so its quite an early one and i had a good look when it was in for an inspection recently and the mounts look ok. So not sure if they have been done before but its a common problem. The mount splits in half and its obvious.
Either way having the shells replaced is about £1000, while they are in there and have the tray off the car might as well get the other bits done too. Roughly £1500 depending on where you go and what it needs.
For me I have a list to make the car my own to ensure my anxiety is at bay

although I know both the previous owners they both used the car as intended. With that in mind I plan on doing as much as I can to keep it running as it should for as long as possible.
I plan on doing a full suspension refresh and all of the above at 80k which is in 15k. Simply because I have decided it will be a long termer. At 13 years old im sure the ride and handeling of the car would benefit.
My old M coupe had lowering springs but with the same wheels felt more direct, im not sure if its a roadster thing but my MR feels a bit more woolly in comparison. It could just be that its 6 years since I sold the coupe and have had lots of other cars in-between and im not used to it but it seems to need a decent turn of the wheel to make small adjustments. I feel like an upgraded suspension would benefit hugely.
Again personal preference if the car isnt something you will keep then you may be different.
The main thing is just keep running the car. They are designed to move and sitting does them no good.