3" downpipe will fit both ko3 and ko3s. As for your other mods it really depends on the kind of power you're aiming for.
Before you do anything make sure the engine is running well, as a minimum I'd do an oil/filter change and check for split boost pipes. When were the cambelt/waterpump last done? Waterpumps are renowned for failing. Oil pickup pipes also get blocked so is worth replacing if there is no record of it being done.
Once the car is running ok then look at mods, either forge d/v will be fine, TIP is a must, personally I'd get a full hose set as well, forge and ths intercoolers fit well, alternatively search for 'wellycooler' and take a look. I'd also think about air intake, again there's a comprehensive thread about if already if you search.
What power are you aiming for?