Congrats on the purchase, you'll absolutly love it!
Now, I have 338K on mine, and holding strong, a few others have much more.

1) no (no justifiable reason to hold off on a 4" DP, better flow, lower EGT's)
2) no
3) 18-22, 14 loaded
4) plenty of good suggestions, follow with what ya like, and I'll jump on the band wagon too here. GUAGES

Here's my current set up:
4" Rip Rook downpipe, 5 inch stacks
fuel scew maxed out
3200 RPM spring (BEST mod EVER!)
star wheel bottomed
old smokey grind fuel pin
timing modified
"smoke screw" 1/2 turn in (going to go 1 full turn and leave it)
8/18 valve adjustments
open element air filter setup
autometer phantom pyro
With all that, I run 18+ MPG. Also, my cruising EGTs are around 6-700 depending on speed, air temp, and hill or flat. I pulled a 2 car Kaufman gooseneck from buffalo NY last night and never saw over 900* (due in part to a free flowing exhaust!)

In the box at this moment, in my living room:
WH1C
and 14CM W/G housing coming soon. (VERY soon! )

 

I don't know why anyone would keep that garbage stock exhaust. It benefited me huge even with stock injectors. It all comes down to pumping losses. Yes you need to burn more fuel to generate more power but reducing pumping losses will eliminate parasitic losses of power which will result in more power at the tires. I noticed it big time with my truck. 4" turbo back is a must.