I have about $800 in the mods to my truck. Simple... gauges, summet sells a A-piller pod and gauges (boost, egt, trans temp) that you need. Not a hard install, lots of advice here on that... Intake, get a wix 6637 (I think that's the part number, correct me on that if I'm wrong) air filter and the DIY intake mod will take about a half an hour with the filter and about $5 in hardware. Exhaust, straight pipe it, ditch the muffler. Not obnoxiously loud (my trucks are company trucks, painted/lettered, trying to keep a professionial look) and mine was free, I had the pipe and welder in the garage. A shop shouldn't charge much... $10 in pipe, and less than an hour??? The stock exhaust is fine for up to 400 HP from what others who's option I trust have posted here, the 'bottle neck' is the muffler. Once that restriction is gone, you need major mods (injectors, big turbo, etc) to really need a huge exhaust.
Once you get all that done, think about a tuner. I have a DP Tuner, almost 2 years now. I really like it... there are also other good ones out there, I have not used others. Meny will say to not waste time with anything else, just get a good tuner DP, TW, etc. I did it, and never looked back. Run my truck daily on the 80 HP econo tune. Tap a button, and I got 40 HP tow tune... so easy. Down hill, tap a button, I got a decelleration tune... great for towing, saves over heated brakes.
Best of luck with your new find... that truck should last you a long time...
'99 E350 DRW Stahl 11' util box van, 7.3 PSD, stock. 138k SOLD
'00 F350 extcab, 9' utility bed, dually 228k
E'99 F250 extcab, S/Bed 211k. Sidelined due to motor issues.
'00 Excursion Limited, 7.3PSD 294k
'84 Chevy K-30, 6.2L TD, woods/trail beater. 141k