Ironheads make nice chops, i've done 3 already. They are nice because they are so inexpensive and parts are still very readily available. I've learned some tricks to minimze the oil leaks on the top end. The drain plugs get gubbered alot under the crank, check this plug as this is a main source of oil leaks on these. AS far as reliabilty goes, i have not had any issues with mine, or the one i did for a friend of mine. I threw out the factory wiring harnesses and made my own from scratch. S&S carbs are well worth the money, and once you get it tuned and the ignition set up, they run great. mine were all kick only, and would start in 2-3 kicks. I've picked up all my ironheads for under 2 grand to start my build. I would love to do a panhead myself, but you just can't find them reasonably priced. Myself i wouldn't want to cut up an original pan, i've looked for engines separtely but they are pricey too, then you have to get the trans, the primary, it all adds up rather quickly. IF you'd rather go with a big twin than sportster, a shovelhead or an early evo might be a good option. You can pick up mid 70's shovels for 3-5k and be off to good start. The shovelheads are good motors, one of there flaws was the engine oil is circulated over to the primary and back, so all the clutch debris gets mixed in with the oil, but that can be changed, i did it on mine and my buddies FLH.
Any of these old harleys are going to leak some oil, thats all there is too it. It can definetly be minimized though. Basically your pocket book will determine what your gonna build.