I have a 93TA A4 with 85,000 miles on it. Will i be able to handle a 75hp shot? What gains in the 1/4 mile will i see?

Yes, you should be fine.

I had over 100K mile on my '94 Z-28 before I started spraying.

My old car:

'94Z With (105,xxx)
Mods: 3:73, Girdle,
3000 TC, GMS MAF,
K&N CAI,Free MODs,
MAC headers,OR pipe, 3in cutout flowmaster muffler
160 stat,meziere electric WP
30lb SVO's, MAC SFC,BMR STB,LCA&RLB's
MSD inline fuel pump
Nitrous works wet plate system (75shot),

N/A(3:73)13.251@103.41 60ft 2.003 on BFG DR

N2O(3:42 stock TC)12.679@111.74 60ft 2.028 on BFG DR

GOOD LUCK!

Should be no problem, I started spraying at 90,000 miles.

As far as iv'e researched, it is totally safe to run a rather small 75 shot as long as you invest in all of the safety equipment. I am shopping around for some goodies for my setup as we speak!

My had 50,000 miles on it when I started spraying. However, I did lose an engine. I made the mistake of not checking for misfire when I knew there was a problem, so that was my fault.
Anyway, I use a 75 shot on a 4 banger. It may not have high mileage, but with all the right safety equipment it should be safe for you especially since it is safe for me. Just make sure you do it right and check everything out. Check for misfire with a diagnostic program. I use AutoTap. If you don't have that, a quick check at a fave dealer/mechanic will do. Or a friend's...

It should be safe for you. If you decide to go with a bigger shot you may want to invest in some larger injectors, fuel pump, and an adjustable FP regulator. Running with higher octane gas is also a plus.