Building Scout will seem a little odd at first. Since the team doesn't rely on your damage, they rely on you staying alive. In this case, you will build him like a tank (mmo slang for the "tough guy" of your group; all defense, no offense). If you start on a Mecha Engie mission or any mission that gives you like $1,000 bucks, then the first thing you need to max is Crit Resistance. It totals to $450 for the full upgrade. On harder modes, this is vital. You will eat some critical hits and the average crit does over 300hp damage. 125hp - 300hp= -175hp so that means you are more than dead in one crit. Max crit resist will give you a 90% damage reduction from criticals. Don't get too confident; crits will still do a scary amount of damage because it doesn't reduce the entire 300 damage, only the crit part.
Let me explain: a normal rocket will do about 100-120, but a critical will do nearly 300. A crit adds 300% to the damage, (I suck at math) meaning 100 x 3 = 300. If you have 90% crit resist, you take 90% away from the total.. It would look like this: 300 - 90%= 30 then 100 + 30 = 130; the official damage done. Keep in mind these numbers are really rough estimations just to give you an idea of what the heck I'm talking about. The point is: even with crit resist, you are still getting hit really hard! It allows you to limp away from sustaining a few. It doesn't seem like it helps much, but believe me; it helps a BUNCH, especially when combined with...
Blast/Bullet resistance! Have you ever heard this bizarre SHING sound when you are hit in MvM? That sound is indicating that you are reducing damage taken from that source due to resistances! At the top of the screen, you need to see what bots are attacking next. If you see a lot of Soldiers, go for blast resistance. A lot of Heavies, go for bullet resistance. If you see neither, it must be an easy wave, so spend very little (maybe one Uber Canteen).The only one I tend to either ignore or put off until later is Fire Resistance. If you stay out of Pyros' range, there's no need for fire resistance. The afterburn will be countered by my next favorite: Regenerate 2hp per second. Later, you will want about 4-5 points in health regen. Scout only has 125hp and with resistances reducing the damage you take to about 30 per hit, you will regen most of your health DURING combat. This is my secret to soloing a giant Heavy bot: resistances, mad milk, FOW, soda popper, and health regen. Isn't it beautiful?
There will come a time when defenses won't save you. That time is tank time! When the tank shows up on the "next wave" bar during the upgrade session between waves, it's time to switch to soda popper. "Upgrade damage, right?" Not quite. You need damage, yes, but here's a trick I use (math again. ugh...) You have an option between upgrading damage by 25%, or add a bullet to your clip. If you upgrade damage: (estimations) (100+25%) + (100+25%)= 250, BUT if, instead, you add a third shot then: 100 + 100 + 100 = 300! So actually, buying a third shot yields more DPS! Later (by wave 4-5ish) you will want at least one point in Ammo Capacity, so your reserve lasts longer and ammo packs are now worth more because ammo packs refill ammo by %. Higher max=bigger %=more ammo per pack...not that you should be getting ammo packs. You should be rushing the dispenser. If you can't get to a dispenser, share the ammo packs with your team. They do more damage than you, so try to go for out-of-reach, small ammo packs and leave the bigger, easy-to-reach ammo packs for your team.
I've recently tried out Jump Height, and I got to say, it is really nice! Your evasion skyrockets with it, and it only costs $300 to max it out! Also on larger maps, Run Speed upgrades are really nice too. It'll take experience to prioritize your upgrade build as there is no blanket, pre-set way to build any character everytime.