Your situation in the US is not unlike Australia, for Mercedes at least. Manuals are very rare here (I see from your signature you enjoy manuals). The very reason I bought the 190E a few months ago was due to its rarity being a Sportline (possibly only 15 in Oz), a 2.3 (most here are 2.0 and 1.8 and the occasional 2.6), and the fact it was a manual. I really have been enjoying driving a manual again and it's ironic that the first thing in the car to let me down is due to it being a manual. At least it will be less costly to repair than an auto transmission rebuild would be.
I guess my main concern with doing the job is that I remember when I was looking under the car at the time of purchase, I noted how tight the clearance is around the transmission and bell housing (ie. for access to bolts securing trans to engine). My Haynes manual also makes mention of this and the fact that it may not be possible to leave the slave cylinder with the car due to insufficient space to separate it from the transmission.
I am familiar with the two-piece driveshaft (tailshaft) setup and have done a transmission removal and replacement (and centre bearing replacement) on my SLC some 15 years ago now.
Regarding the rear main seal, according to the Haynes manual, it does not appear to be a big job to replace once the transmission (and obviously flywheel) are removed. I suspect it may be leaking very slightly since there is the occasional drop of oil from the bottom of the bell housing. This would obviously be the time to replace it if required. I don't believe it to be the cause of the clutch slip though. I suspect the clutch is just plain worn out and I will check it with the gauge shown in the manual this weekend. Likewise, if the transmission front seal is leaking (I don't believe this to be the case) it appears to be simple to replace with the transmission out.
Like most people, I hate working on my back under cars with confined spaces, particularly with large, heavy or awkward parts. I also hate getting half way through a job only to find I don't have the necessary tool or part to complete it. I do my own work simply to save money and know that if it is not done right I've only myself to blame.
Thanks again Larry and I'll let you know how I go with it.
107.023: 350SLC, 3-speed auto, icon gold, parchment MBtex (sold 2012 after 29 years ownership).
107.026: 500SLC, 4-speed auto, thistle green, green velour.
124.090: 300TE, 4-speed auto, arctic white, cream-beige MBtex.
201.028: 190E 2.3 Sportline, 5-speed manual, arctic white, blue leather.
201.028: 190E 2.3, 4-speed auto, blue-black, grey MBtex.
201.034: 190E 2.3-16, 5-speed manual, blue-black, black leather.