My Honda Civic sedan will be 7 years old next month and is going for 90,000 miles. It still runs fine, but I'm looking at some major maintenance at the 100,000 mile mark like timing belt, transmission overhaul, etc. Should I buy a new car or do the fixes?

Does the transmission really need an overhaul after only 90,000 miles?
I owned a 1993 Civic that I kept for 14 years and 235,000 miles. Civics are reliable and I would advise you to keep yours.

Well, what else does it need work on besides the transmission and timing belt? Honda Civics are great cars even with a lot of years on them and even with a lot of miles on them. I would price and estimate the total amount of money it will cost you to fix it all first. Hondas can be worth the work and money most of the time, but whether you should buy another vehicle is totally up to you. If you haven't had any frequent problems with this car before, I say keep and fix. If you have had frequent issues or begin to... then are better off investing all of that money you are spending on fixing it, and do a down payment. Just depends. I hope that even if this isn't the answer you are looking for, that it does give you a little to consider when deciding.

i would make friend's with a good mechanic and keep the car another 5 yrs or until the motor dies or it falls apart

Unless you feel or know that there is something wrong with your transmission then you shouldn't get it overhauled. That's throwing money at a problem that won't creep up for another 100K miles or more.
As for the timing belt. Get it done, if you don't then you will need to buy a new car after your valves pierce your pistons.

Personally I would keep it. If it's been a good car for 7 years why not keep it another 7? I would pay the few hundred they are charging you for the timing belt and drive it another 100K. Plus it will be cheaper in the long run.

I would do the timing belt and keep driving it. Just fix things as they start giving trouble. I've seen them go well over 150,000 and it doesn't have much for trade in/resale value at this point.

First what year is the Honda?