1) You can't go to Horizon until you've picked up Jack, Grunt, Mordin, and Garrus. And once you do have all of them, you can't do anything until you go to Horizon and complete the mission there. After that, I recommend getting Samara, Thane, and Tali as soon as possible. Thane is ridiculously useful against Collectors (and has a sniper rifle, so equipping him with the Incisor gives him basically 100% headshots), Tali is ridiculously useful against mechs (and, on higher difficulties, all mercenaries), and Samara is useful against husks and for setting up Warp detonations. In addition, you pick up weapons on all of those missions; the Tempest and Viper are very handy, the Scimitar somewhat less so.
2) The only thing that makes upgrades cheaper is charming/intimidating the vendor for a one-store discount. Plus, it's not like in ME1 where you could basically buy anything anywhere; there are only five of each upgrade in the entire game (six for some of them with DLC, and seven for the AR), and each one is only available in one location, whether by salvage/research or by purchasing it. And since weapon upgrades do more than anything, including ammo powers and suit parts, to increase weapon damage, they are highly desirable and should be acquired at the first reasonable opportunity.