[Paul Harm]

I may not be in the good graces of some but what I'm going to write has worked for me since about 2005 shooting Damascus barrel guns. Been reloading since 1970.
You can shoot 2 3/4" shells in a 2 1/2" chambered gun - the pressure only goes up about 500psi. Sherman Bell did test about this and published them in the DGJ [ not in business anymore ]. He also tried to blow up in two different test, 40 Damascus barreled wall hanger SxSs. He took one Parker to 30,000psi before it let go. NONE of the wall hangers would blow with Remington proof loads of around 18,000psi. The reason I'm writing this is I believe you're worrying too much about shooting your gun. Most guys like to keep loads around 7500psi, but 8 or 8500psi loads wouldn't do any damage. I personally like to keep mine under 8000psi. If you call Alliant or Hodgdons I believe they'll still mail you one of their reloading handbooks for free. I find them much easier to use than the internet.
A Mec 600, or Jr as they're called, is probably the best bet for the money. A used one should still be found for around $100. Figure in a powder scale [ electronic for $35 ] from BPI or Precision Reloading.
Look up reloads for target shooting with 1 oz or less and you'll find many loads. The big problem is finding powder and primers. I lump primers in two groups - mild and hot. The Fed 209A and CCI 209M are hot. The rest mild. If you're reloading at say 7000psi then primer subing with in the two groups is irrelevant. Pressures won't change enough to make a difference. Cheddite primers are about the only ones you can find right now. So don't worry if the data calls for Win, Rem, Noble primers - they're all mild.
Shells in 12ga are either straight walled or tapered. The only tapered wall hulls are Win or Rem. And some new Federal hulls because they bought Remington reloading industry and label some of their shell Fed, but use Remington machinery. Most guys use a SW wad in a SW hull, and taper wad in a taper hull. The tapered wad will fit in a straight hull but you'll loose pressure. Straight wads won't fit in Tapered wall hulls.
As Brian said, with pressures in the 6 to 7000psi range you have a little leeway if the load is somewhat higher. Enough said - good luck. Find your powder first and build your load around it.