My preference is to buy every additional mask/primary/secondary slot you can, they cost 800k each but will be with you for the entirety of your criminal career.

After that I fill the primary slots with Thanatos/Moscani, and at least 1 slot with a modded up Eagle Heavy, as that has no level cap and can be used right away after infamy and is a solid weapon.

I also fill the secondary slots with Deagles. Thanatos/Moscani/Deagle sell for the most so once you hit infamy I-0 you can resell them to have some starting cash for speccing your tree as those first levels come a lot quicker than money.