There shouldn't be any erosion problems in your steam system, unless there are big velocities combined with ugly steam quality. There shouldn't be much in the way of internal corrosion either, in a properly designed system.
Strictly speaking, Sch 40 A53F pipe and 150 malleable fittings will likely meet the code requirements. In practice, I'd use a better grade of pipe because the quality control on "F" pipe is often just horrible, and we've hit a lot that (supplied by plant owners) just doesn't thread very well. I strongly suspect that it has a high level of remelted scrap in it, and it's not mixed very well. We normally buy A106B because it's a good grade of commercially available pipe, and the nipples are stamped "A106B". Lower grade nipples aren't identified, so you're pretty much left with taking someone's word on just what you've been handed. For anything 2" and under to be threaded, we use Sch 80. It has a lot more mechanical strength to deal with abuse like being hit by lift trucks, used for hanging chainfalls from it, etc. (These things WILL happen in most general industrial plants.) Only having Sch 80 on hand eliminates having Sch 40 making a "surprise appearance" in condensate services and other places where it's not permitted by code and/or it's just wrong from a maintenance standpoint.
150# malleable fittings just seem to keep getting lighter and nastier. We do our best to avoid them. The 300# malleable should be fine for what you're doing. The actual threading of the pipe itself is usually the source of problems with screwed joints leaking. Just because somebody has a couple of pipewrenches in their toolbox, and access to threading equipment doesn't mean they know how to properly thread pipe. A huge number of problems stem from not knowing how to set up pipe dies (particularly the adjustable ones). They should be set to give at least 3 turns, but not more than 4 to hand tight, on clean dry threads. The lenght of the thread should be the same as on a factory pipe nipple of the same size. We give our threads a quick rub with a wire brush to remove any little chips, apply the heavy orange or pink Teflon tape, then use the edge of the spool to chase the threads, pushing the tape into the bottom of the thread root. Stay back one thread with the tape, when using properly set dies, don't overtighten the fitting, and you'll never have strings of tape dangling inside the pipe. Tighten the fittings on using the appropriate sized wrenches. DO NOT let anyone use the power threader to tighten the fittings on by jamming a pipe wrench onto the carriage guides. The joints get made up too fast, which generates a significant amount of heat, often resulting in leaks. This is also a prime cause of overtightening of fittings.
Threaded joints unfairly get a bad rap and it's not typically the joint itself that's at fault - it's usually the untrained guy putting it together. The main thing to remember is that merely possessing a pipewrench doesn't make somebody a steamfitter any more than possessing a claw hammer makes them a carpenter. We have put together literally thousands of 2" and under threaded joints on steam, feedwater, blowdown lines, compressed air, etc in the last year alone. I can't remember the last time we had a "leaker".
