Don't think there's much point just doing valve stem seals. They probably got knocked out due to loose valve guides. It's probably worth waiting and getting the exhaust valve guides done at least.especially with high mileage. My engine had loose exhaust valve guides at 123k when it was rebuilt.
Usually about £15 a guide. you can lap in the valves, decoke etc. Put the stems in yourself with a deep 10mm socket. You may get some power back also.