I have both a Mec 600 Jr. and a Lee Load All. The Lee is functional but deffinitely a sub quality set up. On the other hand, a used Mec. 600 Jr. isn't expensive by any means and can be found quite easily, especially now days. With shot being so expensive, many shotgun loaders are getting out of the hobby. I still load a box for about $1.60 because I have found sources for reclaimed shot and inexpensive wads. But my personal recomendation if you plan on using it a good deal, is to buy the Mec 600 Jr.. Even new they aren't that expensive. Once you've done your home work, you'll have a good idea which powder bushings you'll be needing to get started, and they're only a couple of bucks each. I don't know anything about loading steel, so I can't help there. But if you are going to load 2 3/4" shells a standard M 600 Jr. will get you going. I think the newer Mec 600 Jr. will load 3" without any conversions. Mine is almost 30 yrs. old and won't load the 3" shells unless I buy a different sizing die I think. But a testimony to the quality of a Mec 600 Jr. is mine. After almost 30 yrs. of extremely heavy use it is still as relaible as the day I bought it.
A used M 600 Jr. can be found for $50 with a little searching. Often times the guys selling a used press will be throwing in a bunch of hulls, wads, powders, powder bushings, shot bars, and what not. I saw one a few weeks ago for $125 and it came with just about everything you'll ever need.