Try to look for an R53 from 04-06, If you choose a sunroof, check it to see if it opens smoothly, they are expensive to fix if not maintained. 150-170,000 km in Australia would be considered mid mileage cars. Issues with the R53 as they age like the R50 would be a tendency for the timing chain tensioner to start sticking but its an easy DIY fix. Worn engine and gearbox mounts, again, easy repair. Due to the redesign of the front strut tower mounts, the towers can be domed (mushroomed) due to hitting large potholes and worn shocks, new suspension (shocks) and under tower indurators ( Under Tower Indurators - CravenSpeed Online
) will fix this. Check for leaks around the PS Hoses, leaks around the Thermostat housing, Oil leaks around the crank sensor housing (easy fix, on the front of the engine and often diagnosed as a leaking sump gasket). Can sometime leak on the oil heat exchanger (oil cooler) and allow water into the oil system and vice versa.
What you will be doing if you buy an R53 is change the engine oil and filter, air filter, serpentine belt (need a tool to release the belt), spark plugs, gearbox oil (every 50,000km), change the oil on the Super Charger ( every 50,000km) and It has two reservoirs and requires to be taken off the vehicle, again a simple job. Whilst your changing the gearbox oil and SC oil, you may as well replace the coolant, use the BMW stuff as its not expensive in a 50/50 mix with deionised water.
A very good source of information, especially for DIY repairs is Modmini on YouTube for his Gen 1 videos. If you have an R53 and your looking at a new clutch, a Valeo Single Mass conversion is good value for money and is covered on Modmini's videos.
A rattle on cold startup will indicate a minimum of a sticking timing chain tensioner (mentioned before) or may indicate new timing chain guides (requires a camshaft locking tool and Modmini covers this in his cylinder head gasket replacement). Very robust engine if it has not been flogged to death and reliable, like all engines regardless, does require regular servicing and maintenance, a lot more frequent than the BMW schedule but will benefit with a reliable engine. Any issues nipped in the bud early will see cheap and easy repairs, left to fester will result in greater expense.
I own both an R50 and R53, 02 and 06 respectively and both are over 200,000km. Yes parts wear and I do all my own repairs and service and have saved a small fortune. If you choose a Gen1 Mini, consider buying the diagnostic cable (blue) with software from BMW DCAN OBD USB Diagnostic Fault Code Cable - This Lead allows full Coding Programming and Diagnostics for your BMW D-CAN
, the software works best on XP based computers (laptop) but will work with a virtual machine on systems upto win7.
Also consider buying the Bentley Manual for the Gen 1 series as it is worth the expense in the long term.
Best of luck