Hello there, I would suggest you to defenitely buy the Mislle rack 8 as its really good. Since you are a new player, don't upgrade your common mechs much(i refer to paragon and lancer and M.D specifically). Focus on acquiring mechs like killshot, Ares, Juggernaut(or upgrading then a bit{upgrade them 1 rank at max and then upgrade based on playstyle} if you have them already)
Upgrading mechs is dependent on playstyle, If you are more of that attack and end enemy type player then upgrade attacker mechs.
If you are a camper, upgrade scouts and purchase the Javelin Racks
If you are a defender, buy and upgrade tanks(Ares and Juggernaut)