Will to live isn't retroactive, I picked it up after Colonel Lyra Heartstrings tested psi-postive and her will didn't change. However, Scootaloo is the wave of the future and is probably going to break 100 will if I can keep her alive and get her some psi-armor.
Both squad size upgrades are obviously a must as well, and I think that +xp is totally worth it.
Really? I haven't much liked any of the foundry items. You do have several projects to get a good S.H.I.V., but I'm not certain that's strictly required. I haven't done the pistol research either, and that just doesn't seem worth my money compared to just buying more soldier armor and upgraded main guns and armor. Improved arc thrower was nice, and I haven't researched stealth satellites, but I'm a little worried that failing to get that might screw me later.