Need to repair radiant heat before winter

Hello,

I'm new to radiant heating (previous houses I've lived in have been forced air or electric) and I need some advice for getting my system operational, and I have some specific questions as far as repairs and upgrades.  I apologize if this post is too long:

I bought a foreclosure that had BOTH an improperly winterized heating system and stolen copper.  The stolen pipes aren't a big deal, there's enough left so i can figure out what ran where.  My biggest problems are the cracked radiators, actual volume of work, and that I have no idea if the system ever worked properly.  I have little knowledge about heating systems, but I have decent technical ability and most of the required tools (I think). 

What i have:

Burnham boiler, from '04, model 204PVNI-T2

Expansion tank with air valve on bottom, looks like about 3gal capacity

Taco circulator, 1/25 HP

2 taco 572-2 zone valves

Base-ray baseboard radiators, some of these have visible cracks

runs to baseboards are 3/4 pipe



My thoughts:

Furnace holds pressure when i close all the valves and open the 'fill' valve from the water supply, so i assume its good.

I could probably test the circulator by putting 120v on it, just for a second

I could probably pressure-test the radiators with either compressed air or by connecting a hose to a water supply



My questions:

Pipe coming from furnace starts with large pipe with tees to smaller pipe to each room, should i set it up the way it was, or get a manifold system? I assume i need 2 manifolds for 2 zones?

Is 3/4 pipe for each room standard?  A lot of stuff i've seen online is for 1/2" runs...

My setup goes zone valves -> circulator -> furnas ->expansion tank -> radiators -> zone valves .... is this correct? I've heard that you should pump away from the tank.  If so, pipes between valves, circulator, furnace and tank are intact, is it worth re-configuring?

I've researched the baseboards and found that they are in sections and can be separated and pieced together, i can probably piece together enough lengths for a few rooms, but are these even worth saving?

I've also looked into radiant floor heat for my bottom floor (there is a basement beneath) - is it as easy as running pex and stapling the aluminum plates up against the subfloor?



Any input, suggestions, or thoughts are greatly appreciated

Thanks for reading!