I'd say a stock 245 is not perfect. Built in understeer for safety reasons and the stock US issue "M" cam is pretty tame.

Check out the Grimshaw Volvo Performance book. Forget if IPD still sells it...

I have done the following mods to my 91 245 M47 and it changed the car for the better. Quicker and much better handling.

IPD swaybars
GT firewall braces
Cherry Turbo strut brace
Bilstein shocks
15" 740 rims with wide Michelin tires
mix of poly and rubber bushings
IPD VX cam
adjustable cam gear advanced 5 degrees
Had the injectors cleaned and balanced
Unitek header with turbo single muffler exhaust
Unitek alloy underdrive pulley
940 electric fan with 100 amp alternator
Barton adjustable voltage regulator
Magnicor plug wires
Also swapped out the big steering wheel for the smaller Turbo one.

You can go a lot further than I did.

Engine "must have" would be a better cam plus adjustable cam gear. Volvo factory "A" or "B" at least. VX is good but pricey. Enem V15 is a great cam but even more pricey.

Suspension "must have" starts with firewall braces and better shocks. Bigger IPD swaybars if you can afford them.

You can avoid an unbalanced ride when modifying the suspension by replacing parts in one big blow, i.e. shocks, sway bars, bushings and such.
And yes, seriously, modifying the suspension like that really does change how a 245 drives. Much tighter and more prone to "go where you point it".

Stock brakes are pretty good as is. No real need for stainless lines.

"Beefier voltage regulators" would actually be a higher output alternator like a 100 amp Bosch or Densa (important if you convert to an electric fan) and maybe an adjustable regulator.

Go over to TurboBricks.com and check out that forum. Lots of performance info (not all turbo) and a good buy/sell market.

Lastly "stage 0" means all stock components in perfect operating condition. Most cars need work to get up to stage 0 by fixing what is aged or broken. If things need replacing then why not upgrade?