Gib,
Don't be a affraid of a good, sharp hand saw for cutting tenons. You can buy a decent hand saw at your local hardware store for as much as you would spend on your specialty angle grinder discs. I find a good sharp 8 tpi hand saw will work fine for both cross cuts and [small] rip cuts. Use your skil saw to go as deep as you can and finish the cut with your hand saw. An even better source for a hand saw is an antique shop or ebay; buy a good quality old saw cheap and spend $8-10 to get it sharpened. A well kept hand saw is a fantastic tool and I will choose it over stringing out an extention cord often. I recently picked up a Disston D15 for $10 from an atique shop, spent $8 to have it sharpened and since have all but retired my 7 1/4" skil saw.
As for squaring mortises; it is quite a bit more difficult, but you can get by without a corner chisel and use your flat chisel to square your mortises. Or, as with saw, shop ebay for a good price on a corner chisel. Remember, when your done you can always resell your tools on ebay.... Our own Jim Clark is always offering tools at a fair price as well.
If it were me, knowing what I know, I would buy the right tools, the bare minimum. Use them, do the job right and resell them if you think you'll never use them again. Fine old wood working tools are actually an investment; they don't loose value as long as they are well cared for.
You didn't ask for this advise, but since I'm here and you made mentin of it: you don't expect your project to be perfect since it's your first: that is ok, no one expects their first job to be perfect, but strive for it anyway. I sometimes get the sense that many people think timber framing is easy because you can get away with a lot of error. My dad, a long time stick frame contractor who helps me on jobs occasionally, is always saying "it's just a barn" as I am shaving another 1/32" off a tenon for a proper fit. Barns built in the 1700s are still standing because they were NOT built on the premise "it's just a barn". Just something to keep in mind as you prepare your joinery.
Good Luck, Gib.
ps: forget the angle grinder, they are for grinding and we don't grind wood....
Shaun Garvey
Berkshire Barns
Dalton, MA
