Looked thru the FAQs, and did a search, but I didn't find a definitive answer

My folks buoght a '98 E320 about 5 years ago with 80,000 miles on it. My dad, living in farm country in Southern VA never saw fit to actually use the synthetic oil that Mercedes calls for, and instead used regular Penzoil Dino oil. Now its got 139,000 on it, and it's smoking like a freight train. I'm pretty sure its the valve seals, as it seems to be dumping the oil under decelleration, building it up in the catalytic converter, and then letting it go in huge puffs under hard acceleration.

My question is, is this a job that a fairly competent shadetree mechanic can do (I've done timing belts on numerous VW TDIs), or do I really need to take to an experienced Mercedes tech? I'm trying to help my folks out and save them some dough, but if it'll cost more in the long run, i.e. too many special tools, more time than its worth, etc., I'd rather
just take it to a competent shop.