I would score the 88. I'm sure you are interested in that sexy front end from 87+. Unless the conversion is already done it's going to be alot of work and money to try and make it look like an 87+.
You'll have to change the rear quarter windows(Ugly Louvers), front end, ugly dash etc.... To much cash, you gotta love that fuel injection also
dont get caught up on what year is the best and let that sway you into what to buy. When i bought my first mustang i didnt look at any other cars really, i rushed into it and luckily it worked out for me and i made money off of the car when i sold it.
but when i bought my notchback i looked at a good 26-30 cars before i found that car i still own, it pays to take your time and find the cleanest car for what your budget will allow. bassically what im trying to say is that if the car is clean and well taken care of, you will be happy with it no matter what year it is.
p.s.- 95% of people will scream "DUDE DONT BUY A 93 IF YOU LOOK AT THE PISTONS CROSS-EYED THE MOTOR IS GONNA EXPLODE, IT HAPPENED TO MY UNCLE'S COUSINS BROTHERS GRANDMA" but the truth is that anything can break and there is ALWAYS a reason that parts break... if you are making enough power to burn a piston up, then its time to step up to a built motor
With a good tune the part that will go first in the motor is the rods, not hyper pistons. That said, forged materials are better for handing knock and are just a little extra insurance if you go the boosted or nitroused route.
My personal preference is the '91-'92. I've owned '89, '91, and '92 foxes. I liked the wheel on the '89 more than the airbag one. It had tilt wheel also, but the position of the wheel in the later two years was where I put the tilt in my '89 anyway. The interior light is better in the '91 and '92. I especially like the pony wheels. I think they look sharp on the old foxes. Other than that, I'd probably get the best deal I could find on one regardless of the exact year between '87-'93.