well first to elimate the pistonh rings i would do a compression test, and check the intercooler pipes for oil, also take the cat off the turbo and check the exhaust housing for oil.
Valve stem oil seals are a bit of an involved job really, you have to removes the head strip it right back remove the valves replace the seals and relap the valves. you could maybe buy a second hand head off someone on here but theres no garuntee it will last as these cars are getiing old and stem seals are common on cars 10+ years old.