Ok, I might get chewed here, I might not. To start with. I have a 72 1200 sedan shell, with a L20b and 5 speed strapped into its bay( tunnel cut out) I bought it like this.
I own 2 spare a12s, and 3 4 Speed gear boxes.

I am looking at making the sedan, a beginner circuit/ hill climb car. Maybe even put it on the roads for a bit.
The question is what is an ideal set up to begin working towards.

You could complete the L series install, get z20 front timing case, toyota 22r crank sprocket to suit single chain and Ka24de head, run carbs and good pipes with megajolt like DDgonzales is selling the kit for, get if not done good brakes setup suspension then engineer it to cruise to simple events you choose.

Alot of work ahead of you but some good things already sourced too.
if interested for reference look at some of the L series engines converted on Ozdats site

What kit is this?, I'd be interested.

Search oZdat for ka24de head and L series, grab a hot cuppa and enjoy the info

Great idea. If the car hasn't been setup for the larger engines, put the A12 back in and start racing in the stock class where driver skilz can be honed.

Just bought an a15 bottom end. I'm going to put that and a 5 speed in.( after talking with my mech bro in law)

I like the ideas you guys put forth though..

start from the letter A :) and later progress from there

I'd leave the L20B & box in it. Get some large front brakes & struts (200B or Stanza), & see if you can find a H165 rearend from a 1200 ute or a Borg warner rear end from a Corolla (correct me if I'm wrong please Harry). Some cheap mags from Ebay or a wreckers, 13-14 inch with good tyres, a good spring set front & reset rear leafs with the main spring turned over, good dampers/shocks front & rear, a 2&1/4 inch exhaust, repair any rust & give it a good polish of what ever paint's still there. You could use the auto trans tunnel from a 120Y to cover the gearbox, or you could fabricate one yourself, depending on your skill level. If you go to the A15, you still need do most things in this list. The best way to drop the A15 in without too much trouble is to use a late 1200 ute crossmember & engine brackets, the gearbox crossmember needs to be made. Don't forget to browse through the photo album, as lots of little tricks & solutions have been posted there over the years. Anyway welcome to 1200.com & we always like to see photos, so don't forget to post some of your own.

+1 for the L20. More power available for less effort/money. It will take a very good and $$$ A15 to keep up with even a mild L20. And For hillclimbs in particular, the extra torque of the L20 will really help. And the L series have strong reliable 5spds.

sway bar from b210. cheap, effective upgrade.