First - Welcome to the .org! We enjoy all oil burners here - even if they don't burn it in a PSD
But - glad you decided to check us out...
here goes on trying to answer your questions - I am by far not the most knowledgable, but do have a lot of time a 3:45am here at work
Since it appears you will be looking at 7.3's in that range - you are right that they are pretty solid engines. The early 7.3's had forged rods until 00 or 01-ish -- no one is really exactly sure when they changed and the only real way is to look at the rods ...they were changed to PMR's from forged after 00 or 01 sometime..
they can handle usualy up to 400 HP (the generally accepted threshold) without much trouble at all -- There are lots of tuners and programmers out there -- edge, hypertech, bully dog, banks, bla, bla, bla -- but one of the best customer tuners out there for the 7.3 is Tony Wildman at www.totaldieselperformance.com
The manual tranny seems to be an expensive fix compared to the auto tranny (stock comparison) - just ask one of our members (Ditty) and he will have more info that you want about the manual trans. Seems you can get a solid built auto trans for 4K or less.
but just like on your cummins, the trans is the weak link in these trucks too unless you do things to change that like a valve body, torque converter, and extra coolers...
all in all, the 7.3 isn't the fastest dog in the hunt, but will run along nice and steady and still will put the hurt on lots of things out there with some decent mods
good luck with your decision and I hope I gave ya some useful info -- if not, some of the other members will be along to correct me/add to what I've typed
again - Welcome to the .org!!!