Tom A has been very exhaustive. I can say, that in terms of feel Leicaflex SL is even more solid than Nikon F2, however the main advantage would be in the really nice VF.
I would say, that if you think about an SLR with the eyes of a leica shooter, unless you want to be really into long lenses, which I'd think is realm of AF cameras nowadays, what you will want is a 60mm macro, and a 90 and maybe 135 for portraiture.
The strong points of Leica R old school, are the Macro Elmatrit 60/2.8 and Summicron 90/2, plus you also have Elmarit 135/2.8 which is very nice. These lenses at current prices are total bargains. Nikon on the other hand, has a fantastic 105/2.5 QC, not to mention the 105 and 135/2 DC, but the last two lenses will be more expensive than Summicron 90 or Elmarit 135.
If you want to stay on the cheap, look for an F2 with the simple DE-1 prism, the 105/2.5 and perhaps 50/2 HC. If you want to get into Leica R, for me an R4S2 is more logical as a body than a Leicaflex - if it breaks, you just look for another one. I bought a Leicaflex SL2 in beat up condition, and sent it to Leica for a possible CLA - Leica has declined service, and sent the camera to a German repair shop. They wanted 650 EUR for a complete adjustment, which is about 4-5 perfectly working R4S bodies...
Final point - as Wilson has stated above, having a possibility to choose at the same price, I would definitely skip the F2 for an F3 - I still cannot believe I bought mine for 250 EUR, and it was NEW, it still had the plastic wrap on the shutter...