I've got an '88 560 SL and the paint is in really great shape. I'm willing to invest in an orbital buffer, any strong recommendations or comments to the contrary (e.g. stay with the manual "hand" process).

Your Thoughts?

Also, what would you recommend for the leather & vinyl, also in great shape?

The manual crowd and the machine crowd will each tell you their own stories. I have an orbital, I use an orbital. IMHO manual IS better overall. An orbital buffer can do a comparable job to manual but certainly not in tight areas. It is good for speeding up a job or giving your arms a rest. The only time I use mine is when I'm doing something that would really be a pain in the butt by hand.(de-swirling a friend's car for instance) Also if I have already done several cars in a row and my arms were gone.
Certain products work better, or should I say you can get better results using a buffer with some products and not with others. Example: Meguiars #9 swirl remover works great with an orbital and can be time/energy consuming manually. In my experience waxing does not seem to be helped in the least by the buffer. I always manually remove whatever I have just applied with a fresh 100% cotton terry. Bottom line; manual is the best, but the machine can help in certain ways and occasionally make things easier.
For vinyl and leather... For the plastic/vinyl/etc I would highly recommend Lexol's product "Vinyl-X". Either apply and leave on for the wet look or buff for a nice patina. Leather is more complicated. For a quick wipe down something called "Leather-New" works well. It is basically a liquid glycerine treatment. It smells like leather should and cleans conditions well enough for a one step process. For cleaning you can use either straight glycerine soap and a boar's hair brush or just a sponge and some Lexol leather cleaner will give you comparable cleaning results. To condition I would recommend Lexol leather conditioner. If you seek something a bit better and rarer I have some stuff for conditioning that will nearly revive "roadkill". It is called "Lederbalsam" and is made by Passier. This stuff is designed for ultra-high end equine tack but it gives unequaled results on any leather except suede. Hope this helps...Lee