Leasing you hand the car back at the end of the term. Can be fairly low cost if you choose the right car at the right time as quite a few manufacturers subsidise the leasing. EG a new Golf R £31k, for under £300 a month or some £35k+ Mercs at similar prices. Cheaper to lease at these rates than buy and suffer the depreciation that all new cars suffer.

Lease purchase - "think" this is like a PCP you buy it at the end. This can make the monthly payments lower but you have to find the cash at the end.

High miles in a new car won't hurt it mechanically but it will devalue more as the miles pile on.

Best financial sense is usually to keep what you've got if it has been relaible and you've looked after it. Continue to service it with more frequent oil and filter changes and put the money you save by not buying new. You might need some of it for repairs but you won't need all of it and you can save up towards its replacement.

If you're going to be doing very high miles and the Z4 is going to cost you say £1500 a year more in fuel than a boring diesel (it will be after the Z4) then you might reconsider.

Finance in general? Unless you have to have a new car regularly I'd avoid it as the interest payments really add up over time. Plenty of cracking used cars on the market that someone else has taken the depreciation on and will give good service and save you thousands.