12k is a good contingency fund, however, how good will depend on how many years you are planning to keep the car?
Normally a well cared for R35 should not be to expensive to repair, but a serious issue similar to the GR6 tranny will eat up a lot of your 12k.
On an average of 12,000 miles per annum, expect to pay circa £2000.00 p.a. for your annual service and a set of tyres + your insurance and road tax and a warranty if you decide to go that route.