The M3W is definitely something you can look after yourself and pretty much all of the issues are well known and have proven fixes.
Most are covered in this forum and once sorted out they are very usable and relatively reliable cars. Many owners do look after the cars themselves and there is a very useful Alternative Parts List curated by Planenut. The basics are engine and bevel box oil changes every 3,000 miles and replacing the cambelt every 10,000. Otherwise it's mainly keeping an eye on the nuts, bolts and suspension/steering joints.
Specifically with regards to the engine, whilst the S&S factory support has stopped there are still parts and some full engines around. MMC are beginning to get parts remanufactured, for example they now have full gasket sets. It's a pretty straightforward and unstressed engine and fundamental to the character of the car.
If you do need support then Craig at M3WServices is the goto guru and he can provide anything from an MOT and service to a full chassis off rebuild with new paint and interior. He can also provide an upgrade to the Centa drive unit to allow the rollers to be changed if required without having to take the engine out (This used to be known as a "Phil Bleazey conversion" but Phil has now retired).
If you haven't already, then subscribe to the MTWC Touring Group Facebook page for an overview of the sort of trips and things we get up to:
https://www.facebook.com/groups/511022087505443
