Look at Modmini's videos on YouTube for common repairs so you can work out the cost of parts and what labour is involved and if the work is within your scope, also factor in specialty tools you may need.
Milage is no indication of how the car is, its how its been maintained period, if its only had service book maintenance I would be trying to find out what workshop repairs have been done to it. A rubber stamp for an oil change wont tell you if its had a new clutch or drive shaft and to be honest, service books are a waste of time as there is no guarantee of competent dealer servicing.
Modmini has a video on preinspection of a Gen 1 mini. If the SC is noisy, its going to be expensive to have it rebuilt but there is a place in the UK that can rebuild and recondition them if they are salvageable.
Pull the lower door and hatch rubber seals, look for rust, inspect the battery well, look for rust and or water. Crawl under the car, look for scrapes, oil leaks, check the front strut towers for doming. inspect the bushing and ball joints. Make sure the Power Steering fluid is a light green and not red (ATF fluid), if its red, walk away or be prepared to fit a new PS Pump. Look around the thermostat housing for water leaks, check the coolant is correct or be prepared to change it. Obvious bodywork issues, check the carpet in the front for dampness as you may have a badly fitted windscreen that leaking. Look at the bodywork gaps for consistency. If the clutch feels heavy, its worn and will soon need replacing.
The Gen 1 PS Pump whines, excessively loud whine indicates low PS fluid or a worn pump. Take it for a test drive and don't fang it, your not out to race it, your listening to its engine, excessive body noises, does it pull to one side, does it idle smoothly, are there any warning lights.
If your happy to pay 6k, make sure you have a spare 3k for unknown issues that may occur (contingency plan), A dealership is no better than a private sale, ask why the private sale is selling, is it a plausible reason ? Whilst lower mileage is desirable when considering wear and tear, a well maintained high mileage car can be better than a low mileage, poorly maintained car.
Do your research on DIY repair costs, again mod mini covers a lot of common issues on his YouTube channel, If on your inspection you spot a potential or definite issue, decide if your prepared to fix it at your cost and use that information to bargain a lower price.
My Gen 1 R53 had a fluid leak and an oil leak, needed a new clutch slave and potential sump gasket, ended up being a pin hole in the bell housing to the gearbox, this also resulted in a new clutch and flywheel. Asking price for the car was over 9k, I paid 6k for the car, drove a very hard bargain and paid an additional 1.5 k in repairs to have a reliable car on the road. A dealership had diagnosed the oil leak as a duff motor and wanted more than the book value of the car to put in a new motor !
Since then I had done several oil changes, change the Gearbox oil (when installing a new clutch) done an Oil service on the SC (whilst the gearbox was out), replaced the timing chain tensioner and guides. Due soon for a full brake service and new rotors, car runs well, has the odd hissy fit but that's just part of maintaining a car. I still havnt paid more than the original asking price for it considering what I have spent on repairs, maintenance and servicing.
Bottom line is, have a contingency fund, buy wisely, don't rush into a purchase and do your research on repair costs and parts. GTT in the UK rebuild Super Chargers if they are salvageable and they are cheaper than buying a new SC if you can find one as any SH units will be unknowns. Last, if the car has been modded extensively, be very cautious, stock is better and you can then mod the car as you desire.