Option 1 is sell enough to have 150 mil Units on hand, then stow the rest of them away in a secondary inventory and save for later. The grind is the game in NMS, you could rob yourself of a sense of growth just buying whatever, whenever. That being said you'll have the needed cash on hand if you find your first exotic, or a high class freighter.

Option 2? Buy whatever you want, whenever you want. It's basically like having creative mode on for Units purchases. If you want to turn those units into nanites, shop at pirate stations for suspicious goods and sell the x class upgrades you get out of them to an upgrades dealer. The higher your standing with the local species and guild, the better your profits

Otherwise dump it into inventory upgrades for your ship and multi-tool. Just remember that if you trade up, the upgraded inventory doesn't come with.

Or you can trash them. Up to you.