Ok, you want a SAAB 900 old generation with some push. 
But what would you buy and what should you do with it. 

But before you do anything to rise the power of the engine, You must make sure that the brakes and the chassie is upgraded enough to handle the power!
It's not just your life! There are other people and animals out there too. So! Advanced driving shall take place on a closed track/road!

Well it's all up to you, but here is my recommendations.

The Car
- Engine should be a B202 T16 
- make it a Aero/SPG. They come fully loaded.
- In Europe, see if you find a 88a (It got the new brakes, water-cooled turbo, and no cat).
- in the US, get a 90 SPG

Widdes Tuning Ladder
Here is my personal ladder of tuning. The following is my steps/levels.

Step 1 (more power for overtaking)
- Sports Air filter
- Mod the AMM (remove the "grille")
- upgraded Exhaust (2.5")
- Tweaked APC
- FPR 3.0 Bar
Should give 185-195 HP (this is my guess, should not be treated as fact!)

Step 2 (Power for the track day, and for some laughs at the Red lights)
- Upgraded brakes
- Upgraded shocks
- Upgraded chassie bushings
- Open Air filter
- Mod the AMM (remove the "grille")
- Upgraded Intercooler
- upgraded Exhaust (2.5")
- Modded APC
- Modded fuel chip
- FPR 3.0 Bar

Should give 195-210 HP (this is my guess, should not be treated as fact!)

Step 3 (No more, MR nice guy. if someone wants to race then they'll need a real car)
- Upgraded brakes
- Upgraded shocks
- Upgraded chassie bushings
- Open Air filter
- Mod the AMM (remove the "grille")
- upgraded free flow down pipe (3")
- upgraded Exhaust (2.5")
- Ported exhaust manifold
- Moved battery
- Modded APC
- Modded fuel chip matched to new injectors
- FPR 3.3 Bar
- Upgraded Intercooler
- Upgraded injectors
- 2.1L cylinder head
- 2.1L intake manifold
- Upgraded TB03 "trim60" or TE05-12B
- Upgraded clutch
- Upgraded flywheel
- Upgraded gearbox
Should give 200-250 HP (this is my guess, should not be treated as fact!)

Step 4 (Will it be legal?)
- Upgraded brakes
- Upgraded shocks
- Upgraded chassie bushings
- Open Air filter
- Mod the AMM (remove the "grille")
- upgraded free flow down pipe (3")
- upgraded Exhaust (3")
- Custom exhaust manifold (Power header)
- Moved battery
- Modded APC or other boost control
- Modded fuel chip matched to new injectors or other fuel injection system
- FPR 3.3 Bar
- Upgraded Intercooler
- Upgraded injectors
- O-ringed Engine block
- Forged Pistons
- Balanced crank
- Balanced rods
- B234 Ported cylinder head
- Custom intake manifold
- Custom Turbo TD04-16T or similar
- Upgraded clutch
- Upgraded flywheel
- Chill cast 4-speed gearbox, strengthened
Should give 250-300 HP (this is my guess, should not be treated as fact!)

Tuning order
- Make sure that everything is genuine SAAB parts to start with. 
- Make sure that it works as it should before tuning. Use your common sense.

Widdes suggested tuning order:
1. Get a sports Air filter
2. Mod the AMM (remove the "grille")
3. Tweak the APC (turn F and P)
4. upgrade the exhaust (2.5")
5. upgrade the brakes (discs, pads and hoses)
6. upgrade the shocks
7. upgrade/refit the chassie bushings
8. upgrade the FPR to 3.0 bar
9. upgrade the fuel chip
10. Modify the APC
11. upgrade the Intercooler
12. add a open air filter 
13. move the battery to the left side of the engine room or to the trunk.
14. add a 3" free flow down pipe
15. upgrade the injectors and upgrade the fuel chip upgrade the FPR to 3.3bar if needed.
16. upgrade the gearbox
17. add a water injection system if needed (due to where you live)
18. upgrade the turbo
Easiest is the TB03 "trim60". It's a bolt on. You could also go with a TD04 18T with the number 7 exhaust housing, but the you need to rebuild the oil and water hoses. And also mod the exhaust manifold.
The difference is that the td04 spins up quicker. But you will save the gearbox on the TB03 "trim60".
Another option is the TE05-12B, which is stock on newer 900,s. This turbo also spins up quicker but it doesn't push the same amount of air.
19. upgrade the pistons to forged ones (if needed).
20. upgrade the cylinder head to a 9k long block version or the 2.1L 900 I16 version (they are the same)
21. upgrade the intake manifold to a 2.1L 900 I16 version 
22. upgrade the clutch and flywheel
23. upgrade the gearbox and add a Quaife Limited Slip Differential
24. upgrade the brakes again (front brake discs 330x34mm, rear brake discs 288mm, racing brake pads, 4-Piston callipers front, upgraded rear calliper)

This would be enough for most of us.

Tuning without money
But what can you do with the car without spending almost no money?

Widdes suggestions:
- Fill the tank with High octane fuel
- Drill holes in the sides of the airfilterbox, to let more air in.
- Remove the rear "grille" on the AMM towards the turbo, to get better flow through the AMM.
- Clean all tubing, to and from the Turbo.
- Clean the IC.
- Polish the intake
- Port the exhaust manifold
- Tweak the APC (turn F and P)
- Exchange the spare tire with a "tire in a can" (to reduce weight)
- Clean out any unnecessary tools in the trunk (to reduce weight)