Core skills will not fail you although nothing in Dust514 is for certain. CCP/Shanghai has a history of changing the wording on features such that they don't mean what they originally did. Normally this behavior is limited to major updates but it is always best to read the description before you buy the skill book and then spend SP on it. Or worse yet, AURUM.

As to drop suits any points spent before 1.8 is released will be wasted. There is no promise of getting SP/ISK refunded and the drop suits will change dramatically. All the bonuses for suits changes, the racial variants will all be out and there is little doubt that something will be OP because nothing that has been released so far has had a gotcha that CCP/Shanghai didn't figure out before the gamers did.

Weapons are in the same basic area at the drop suits. Changes will happen, every patch that has released a weapon has left us mercs with a new OP toy to love or hate. Leave enough SP to go to level 4 for at least one weapon. With that and LP you can get a reasonable feeling for it. If it is too good to be true then expect a Nerf Hammer to fix it, although the Combat and Rail rifles are clearly OP CCP/Shanghai has yet to acknowledge the fact nor indicate they are going to fix them. They seem satisfied to have nuked the assault rifle back to a second class weapon.

Modules? Here I am not sure what you are talking about. Modules are tied to drop suit upgrades so having them separate is unclear to me. Just beware that many skills will allow access to basic, enhanced and complex variants but will actually provide no other benefit while other skills actually increase the effectiveness of the related module. Effectively some require double the SP because you need access to the modules and then a related bonus.

Without a SP refund guaranteed for drop suits, drop suit upgrades and weapons almost everything spent before 1.8 is a gamble. Horde SP and run cheap suits until you can determine if you want to run a cloaked scout suit. That appears to be the favored fit that CCP/Shanghai is delivering as the only solution worth using for 1.8, getting knifed in the back while trying to hack an objective will make most other suits boring. Or too expensive to use.

Good Luck.