Once you've unlocked them get 3 at your main base and 2-3 at each of the others. They're the game-changer as far as air intercepts go. Get Marauders as soon as they become available.After the above, always unlock weapons-tech to improve your interceptors.Skip researching the Corsair you won't need it.Gas grenades are a god-send and if you give stun-sticks to some of your assaults you might be able to grab some advanced aliens where you need to. Next research the aliens to unlock stun technology.Skip Jackal (it's expensive and mostly useless). If you're not going to be able to make it, air-strike it and wait for a better set-up. Only do ground missions close to your first base (air-strike the rest) and also time your drop-ship flight so you engage in the day. I had at least 3 Marauders at each of my 3 bases. I finished with 30 scientists and 30 engineers, all at my main base. Gradually increase both of these throughout the game (I started ramping up as my 3rd base came online). Get 5 scientists at the game start and 5 more engineers when the living quarters at your first base are complete.

You can build AA guns at each base if you like I did but I never used them. I went with North America which was a poor choice I never used it for ground missions and I ended up losing Mexico and South America (my only two lost areas).

Mexico and North Australia / Pacific are good choices. Get 1 x Condor and 2 x Foxtrots at each base.2nd base should be built month 2, 3rd base month 3 (start saving money at the end of month 2).The long-term plan is 3 hangars and 3 radars at each base but prioritise getting a 3rd base up before fully expanding your 2nd base's radar.I got a 4th near the end of the game but that was just because I had so much money :) Cover as many countries as you can with each base. Don't worry about covering land I only ever sent a ground mission to another continent once and that wasn't necessary.

Xenonauts base strategy