I've used Koni Sport yellow struts & shocks on my 4cyl '92 244 for a long time. I think the first spring I tried was 8" x 400lb but the struts were not up to the task. Bouncy. Too soft on rebound. Not good.
I switched to 350lb next, then 300lb. 300lb was probably still too much spring for the struts, but it was pretty decent. That's still in there today...I don't know how many miles, but it's been a long time. I've driven the car pretty hard at Mountain Meet / Tail of the Dragon, and it has done well there. Autocross...not so much....the quick transitions could use a bit more roll resistance.
I think I would try 250lb if you go with 2.5" ID coilover springs. Ben sells the hardware for those (sleeve & lower seat) and whatever other combination of stuff you need. I don't know what length to suggest, so ask Ben.
I started the rear spring rate journey with 11 x 200 stock car springs, many many years ago, paired with Jamex lowering springs on the front, and off the shelf Bilstein HDs. Nice amount of oversteer, but easy to control. It was non-turbo, B230F.
Eventually I switched to revalved Bilstein HDs and 475lb front springs. Then got up as high as 325lb rear and that was too stiff. Settled back to 250lb as my autoX rear spring rate of choice (11", stock 240 upper spring seat used on the bottom, cheap stock car 5" springs)
My revalved HDs were nearly the same spec as Dave Barton's here:
https://www.240turbo.com/specsheet245.html#bilsteins I think 220/110 front and identical on the rear. Mine was a sedan instead of wagon, so a bit lighter, but with heavier spring rate. Krupp-Bilstein in San Diego picked the rates based on the corner weight, spring rate, and intended use that I told them.
I've used up to 700lb front springs for road racing (24hr lemons, '91 245), but we dropped down to 475lb (take-offs from my autoX car) and I think that was better considering we were limited to 200 treadwear street tires. With R-comps...maybe the 700s could work. 500'ish on the front works well if you're really trying to change directions quickly and don't care about ride quality. You definitely can't do this on off the shelf Koni yellows, though.
Soft rear works well, 150-250lb. I guess I would say about 2:1, front:rear spring rate is what I've liked. 550lb front, 250lb rear on my autoX car in its final iteration. 25mm or 28mm front bar, no rear bar. about 3deg front camber.
We used cut wagon overload rear springs + stiff R-Sport shocks on the rear of 245 24hr lemons car, and it handled awesome with the revalved HDs + 475lb front springs....and at least 3deg negative camber from ball joint spacers to widen the front track width. That was ~3:1. And again, 25mm front bar, no rear bar.
Prior to donating my stiff R-sport rear shocks to our 24hr lemons car (valving here, 400/100:
https://www.240turbo.com/specsheet245.html#rsportshocks ), I used them on the back of a softly sprung 244 daily driver with stock rear springs. The rear of that car was PLANTED. Over dips, the rear did not pop up at all like a soft-rebound rear shock tended to do. Try off the shelf Bilstein HDs + lowering springs or overload springs, and the rebound feels too soft. I used 12x250lb front springs with stock rear springs. Stock ride height. Really comfortable, easy to drive, but still with a decent amount of roll, so not super precise if you try to push it. I used 205/60/15 on 15x7, max comfort. It was a lot better than stock, yet very far from the autoX setup. Very comfortable.
tl;dr -- start with 250lb front springs and whatever you have on the rear. It should ride nice and handle nice. 
