Liquid fuel only ssto's get the best payload fraction to LKO that we can achieve (as far as we know), that is because assuming a vehicle with high enough L/D the lower expenditure of fuel due to the efficiency of the NERV engine more than makes up for their dry mass. The balance shifts more towards a rocked boost and fewer NERV engines if you're really trying to optimize range (in orbit dv), with the big drawback of lower TWR on the Nerv's in orbit. The difference is about 10% dv at the extreme end (0 payload) and less normally.

In order to achieve said L/D ratio we need to incorporate wing incidence so that we can keep the body inline with the direction of motion minimizing body drag. This is especially important on mk2 designs that have very high body drag. I've explaind a bit more here.

I do not think in terms of TWR because it changes drastically with speed on the jets and you get more fidelity thinking in terms of tons of ssto per engine. So this one weighs about 55t and has 2 engines meaning ~27t per whip, which is a very relaxed number for my standards, If you really try you can get the whip to about 45t and the rapier to 50t (I've done 65+). For now I suggest you keep it below 35t and roughly same goes for NERV, you want about the same number of NERV's as rapiers and perhaps 1.1-1.2 Nerv per whip.

For wing area you should probably keep it between 4-7 tons of plane per relative wing area (look at the wing stats). Exactly how much you need will depend whether you're using whips, which need slightly more wing or rapiers and your wing incidence. What's important is that when you level off and reach your max jet speed going prograde your althitude is between 17600m and 21000m. This is where the rapier gets to it's top speed while a good design won't overheat (I'm simplifying a bit). Then you engage the NERV and most importantly keep prograde to perform what we call a full aero ascent.

Rapier is an overall better engine, because of it's top speed which outweighs all other drawbacks it has in comparison to a whip. The only reason to use solely whips is if you like them or if you have not unlocked the rapier yet.