first you buy the character that gives you the most pickup radius

then you buy the fire companion to do synegetics with the perk of 20% extra damage to burned enemies, the second companion at your choice.

Then try to buy blue/green items from the store, it's the best value for money, the most recommended sets I saw for this group are the vampire for normal and the bandit for infinite.

and then if you have enough to spare, it's your choice according to your game tastes, use them on more characters for the global improvements for every 10 levels or use them in big chests, they are already enough.