Reagarding 5th gen Prelude (H22A4) VS RSX Base (K20A1) for boost.
Since your WPH limit is 240, Prelude easily.
Prelude comes with 200 HP stock, RSX comes with 160.
The Prelude’s H22A engine is a true DOHC VTEC engine, the RSX base K20A1 has VTEC on one (intake only) side ala SOHC VTEC. It doesn’t breathe stock for stock nearly as well.
Those 2 factors alone means you will need less boost, and less work to get to 240 WHP on the Prelude. To reach 240 WHP with a turbo is nothing on either engine, and will not really stress the engine. People make 220+ on bone stock D series SOHC engines, and 260+ on non VTEC B18A/B18B engine, have been for years with bare minimum cobbled together home done turbos. The cheapest ebay kit, and a good tune would probably net more than 240 WHP on low boost bone stock long block. K20A1’s going to take more work, support mods, such as a better intake manifold from a true VTEC K20 head, and if similar sized turbo, more PSI.
Overall, I’d give the nod to the Prelude, because 240 WHP is far easily done on it, and the possible price of entry is lower, and you have a car thats better in many ways than a base RSX.
Any of the “OMG K20 awesome power making” factor is out in a base RSX/02-05 Civic Si (EP3) motor. So how easy power is to make on a K20…without that K20A2/K20A Type R/K20Z1 head, you dont get that.
Also taking a base RSX and later swapping it to Type S, you’d need the transmission, and not to mention the trims themselves have quite a few differences, where you’d wish to have had started with the Type S. Like brakes, gauge clusters, ect. Then there’s the necessity of K-Pro. You’d end up spending quite a bit in the end.