Refurbishing 1994 Trek 950 I would like to strip down and replace components on my old 1994 Trek 950 mountain bike, primarily to replace the drive train, brakes, shifters and wheel sets. I would like to go from a 3x8 to a 1x11. What constrictions or constraints do I need to be aware of? Any advice?
At minimum you require:


new hub in your rear wheel that will take an 11 speed cassette.  May be cheaper to buy a complete new wheel, but 26" and 11 speed hub would be relatively rare combination
new cassette, 11 speed
new 11 speed derailleur with the same mount as your current one
11 speed chain
single front chainring and spider and cranks to suit your current bottom bracket interface, which is probably square taper.
11 speed shifter for the bars


Probably wise to replace your shifter cable inner and outer while doing this - they're cheap.

You may need a new bottom bracket cartridge that fits whatever interface your new cranks have.

You can probably continue to use your pedals.



Price all that up as new parts, and you could easily exceed the cost of a nice non-BSO MTB.  I'd strongly recommend you fix up this nice old bike as-is and just ride it. 

There's nothing better than riding your old bike and keeping up with a modern bike, except for passing and dropping a modern bike.