Welcome to the Compression Combustion Addiction Society!
When looking at a 7.3 (Pretty much any year of this engine is good, and the earlier models will have forged rods, the later models will have powdered metal rods, so take that into consideration. I believe anything before 98...?(Not sure) Will have forged rods. What this means to you is this. a PMR truck cannot be taken over about 375hp, as the rods wont stand up to much more power than that. The forged rod trucks can handle whatever you want.)
I LOVE my 03 (Door sticker says it was built in 02 tho) PMRs and all. When purchasing a truck, look at the mileage (even a truck with over 100k is acceptable if its body is in good shape and its running smooth), and INSPECT THE OIL PAN FOR LEAKS/CONDITION FIRST! (The engine must be pulled from the truck to change a leaky/damaged oil pan/gasket. thats a biggie.) After that, check everything else under there and in the engine bay that can leak for leakage. Start it (It should crank shortly. Extended crank times is a red flag.), listen to it (From the cab, and from right in front of it with the hood open) It should sound like a big loud industrial sewing machine, or a really smooth quiet farm tractor, with no surging, hesitating, balking, skipping, missing, excess clanking, ect. After that, just check it out for generally good condition. I recommend getting an OASIS report on it, and CARFAX it, even if the previous owner has all the maintenance records.
Oh, by the way, the 7.3 is not going to ever be the "Fastest" Without you dumping truck loads of cash into it and basically turning it into a mutated version of itself. HOWEVER. It WILL pull the entire Earth out of its orbit around the sun if you want, and last for nearly ever.
Once again, welcome.