There are only a couple of manufacturers of slaves so it really doesn't make any difference. I get mine from AutoZone or Advance auto. I know the PITA of tranny pulling. I've had mine out 8 times. I'm getting good at it!
Your clutch line or shimming might be your high RPM shifting issue. With a stock clutch I usually say the pressure plate but a M3, although not that great of a clutch for a street car (I had one), doesn't suffer from that much. Even though Monster says you should not need a shim I'd measure anyways.
Changing fluid at oil changes is enough. I found if I drop the remote line down under the car into a pan it works really well by myself. After opening the valve I get it started with a slow pedal push and then just stand there and carefully pour in fluid so I don't add any bubbles and keep the reservoir filled. After cycling through what I estimate to be 2 or 3 reservoirs of fluid I close the valve. No need to vac it or do other bleeding.
I got a solid SS clutch line years ago that replaced the stock one top to bottom with just a short braided hose like you proposed in the middle and a threaded in piece at the slave. If you have the ability to flare the stock line it sure would make it a lot easier as getting that thing to start threading at the master was a hand shredding challenge.
After having a leak at the slave one time I now pressure test the slave by putting it in a vice to keep the piston from popping out and pushing the clutch pedal before putting the slave on the trans. It only takes a coupe of minutes and assures me I won't be pulling the trans again to fix a leak.
If you replace a slave take it apart. You can pull the piston off by twisting it and you'll see that with pushing on a little tab you can pull it off the cylinder. When I did that on my last one (I'm on my 4th in 53,000 miles) I found the cylinder was really wavy where the piston seal rode from clutch dust abrasion. BTW IMHO slaves blow out their seals a lot more with heavy clutches like the M3. With the heavy PP springs it puts a lot of hydraulic pressure on the seals. My latest clutch is a McLeod RST twin. It is day and night better than my M3 was and with light PP springs puts a lot less stress on everything yet can hold 700 RWHP