I've done way too much research into suspension for 156's. I think the fact you don't want to spend too much money, your options are fairly limited. If you go on most of the Motor Factors websites, you'll see a lot of suspension manufacturers, but not many that do stuff for 156/GT/147.
You could get KW V2 coilovers, they'd be around £1100, you can adjust the damping and ride height.
Bilstein B16 coilovers, maybe around £1500, you can adjust damping and ride height.
I think your cheapest option would be Koni Sport adjustables, probably in kit form with HR lowering springs. Trouble is you don't want to go too low so you might want Sportpack springs (about 20mm lower than standard) with the Konis. You'll probably pay more buying springs and dampers separately. I can't think of any other adjustable dampers, unless you pay shed loads.
My GT on KOni FSD's and pro kit springs is incredibly good, even with 18" wheels. I'm just having the Bilstein B12 kit put on the Sportwagon, the kit is probably worth more than the car and it's not adjustable so I'm screwed if I don't like it.
One thing I would say is try and budget for a front suspension rebuild, it's not that much cash and combined with new shocks and dampers, should feel like a new car, well, nearly, I hope.