I'm looking at F-Type R's 2015-16. Would you advise against one with around 35k miles? Is there any maintenance costs/potential issues I should know about?

 

I've had a 2015 R and haven't had any issues with it at all mechanically. I had some stereo issues which have been well documented on the boards, but that was resolved last year. Outside of that, it has been a yearly oil change and service. And tires. I've got about 15k miles, but could use it comfortably as a daily driver if I wanted (I did for about a year). Great car, well put together, and I have no complaints!

 

The major maintenance costs are tyres and brakes, the F-Type and especially the R chews through them fairly quickly.
There are a few niggles with the sound system quality, panel fit and other minor things but no major issues at 35k miles.
At 35k miles it is probably on the second set of tyres by now so they may not be too bad, but they are most likely P-Zeroes which are rubbish on the F-Type.
If the tyres are at or close to needing replacement (make sure you check the inside edges for excessive wear) use that as a bargaining chip to drop the price, and then get a set of new Michelin Pilot Sport 4S - totally transforms the car - with the savings.
Much the same with the brakes, both pads and rotors, either or both may be getting close to worn out, so again check them out and use possible replacement cost as a bargaining chip. With the pads consider getting Porterfield R4S, waaaaay less dust than the OEM pads.