I woundn't get a second hand ECU unless you KNOW what car it came from, how that car performed and EXACTLY what chip it is..... For the little extra money it ain't worth the risk over a new one
If your RST is in good shape, then start with a decent stainless steel exhaust (Mongoose/Magnex/Scorpion) and air filter. Air filters are still the topic of mass debate, but recent tests in mags and on dynos show that an uprated panel filter (ie K+N in the standard airbox (all be it with a little modifying to aid air induction - holes drilled in the wing side of box lid) get better results than con filters, as they tend to suck all the hot air in from the engine bay. Cooler air is the order of the day with turbocharged cars remember
Next you want that chip. If it was me I would get an off the shelf "195" chip, or better still one of Motorsport Developments "Evo" chips in ther, then do away with the Amal valve set up and set the boost up with a -31 actuator. Unless you know what you are doing to, let a tuner set it up.
If your still running std intercooler don't riase the boost limit to more than 12psi tho - 14 is good for S2 RST's as they have a slightly larger intercooler and more importantly, better air flow to it than the S1, and you don't want it on the max 62deg temps all the time.
Get a set of decent plugs and leads on there - a lot of people over look these but they really do help - maybe not in making more power, but they do makea difference to the running
The dumpvalve is another great debate. Personally I run one because it WILL lengthen the life of your turbo over the long run - the point is to expell stuck boost air (between the closed throttle butterfly and metering unit flap) when you lift off or change gear. If you don't have one the air rushes back through the turbo to the metering unit (where it will not be let out) causing the turbo to "stall". htis happens cos the blades are spinning one way and the revers air direction slows or stops the blades spinning. This can have lasting effects on the bearings on the common shaft in your turbo, hence the dumpvalve (which lets the boost air escape before it gets back to the turbo, avoiding this problem) The one you
need is a "twin piston" type - Bailey have it listed as the DV26
12psi, zorst, filter, and a good set up should see you at 155-160bhp which is a good fun figure and really livens the car up
If you want to go further, get some headwork done (open up the inlet and exhaust ports a little to aid flow, along with a good port polish for smoothness) and invest in a hybrid turbo (also good if your turbo is not up to the job) These typically let you run higher boost levels and spool up quicker and dependant on spec will flow more air than a conventional standard affair
Uprate the intercooler to cool the charge air some more. A 4x4 Cosworth one will fit, but mods are needed to the cooler, you will also need a new shortened rad, reverse charge carrier and custom hoses, but gains will he high and 40deg should not really been seen on the road.
Off the shelf you have the GRS and Pro Alloy ones - much larger surface area for much better cooling, and generally fit without any hassle - bolt in affair. You will need new rad fans tho as the old one won't fit unless you want to chop the front panel about.
Raise the boost some more now - 17/18psi and your looking at the other side of 200bhp
Plenty of power for a lightweight FWD car
Hope this helps