The gen 1 is your best bet r50-53 2001-2006
its significantly more reliable than the gen 2 r56 2006-2013.
With a budget of £1500 you will struggle IMO to find a decent low mileage Cooper S so a standard Cooper is a better bet.
Regarding reliability the Gen1 is very strong, however.
2001-2004 One's and Coopers had the Midlands gearbox which is an achilles heel if its ragged or not serviced regularly.
Diff bearings fail, input and output shaft bearing also crumble.
In july 2004 they added the much more reliable 5spd gertag gearbox which is significantly more reliable.
For this reason i recommend you look for a Late 2004-2006 Cooper.
not only did they change the gearbox they also gave the car a minor facelift which is a bonus .
#1 almost all Minis leak usually from the crankshaft position sensor O-ring which is a 30min job with the right tools
#2 they can also leak from Rear and front main seals aswell as the valve cover gaskets.
#3 when inspecting a Mini pull off the door and boot seals and inspect for rust, water tends to hang around the seals and cause corrosion.
#4 Rust isn't a bug bear but have a general root around underneath, consentrating on the subframes.
#5 If it has the brilliant Pano glass roof lift up the mats and make sure the Carpets are dry, the drain age tubes can get clogged with debris causing the rainwater to drain into the cabin.
#6 The BCM "body control module" is located near the floor and its very sensitive to water if it gets water damaged its quite a large bill for replacement.
#7 When you start the car listen out for a Tapping, timing chain tensioners tend to stick after 100k its not a deal breaker but just be aware.
its a very easy job if caught early enough, However i would avoid a car with a knocking engine.
#8 check the Strut tops in the engine bay for "mushrooming" its a common fault, easily dealt with under tower defender plates.
#9 check all the fluids.
Below is a video by Modmini the MINI GOD! anything that needs doing on amini he has done!! https://www.google.co.uk/url?sa=t&rc...Ob8hEn3aCOlJ9j