what is the best way to invest my rank and research points?

(Very) early on, like, first 20 ranks or so I would recommend engineers (man, saying that now feels like blasphemy...), then pour them into decks.

As for research? Go for planetary structures up to Deep Extraction, then get hull, energy, weaponry and sensors.

depends on your rank and what you wana do in game

you will never regret engineers and tactical officers

on research: planet structures - weapons - energy -hull (main things you need)

research wise, everything except cloak is useful, unless u r a hacker in which case cloaking is important too. Thou you will want to do 2-3 tiers of cloak fast and start getting ur blue badges. it's easier now than later
there isn't much point into shield except planet defence structure for a long time, so up those from structure to structure only. Frankly the first few tiers aren't very useful at all, so unless u have some uber amazing finds (VL+ size expandable or M+ non-gas with Rich+ resources in the least) u can skip shield for a while.
Mainly you want planet structure(+6 ASAP, after that u can wait till u feel comfortable to jump to +10, +8 is skippable, not quite worth the CR for mass switching)
Energy(whatever you feel you might need)
weapon and hull(the combat part of your ship, you will want those early to farm out your red badges, much harder to farm later) You can just do weapons after you get done with doms IF you don't like fighting much. You will need weapon for invasion, but hull is rarely useful unless u actively NPC or PvP(low lvl repair bill is cheap so~)
But for all intend and purposes, Hull>Defence for a long time.
Sensors are needed for scanning planet. If you can get into a good legion that frequently share decent planets(VL+ Rich+, L Ex Rich+ etc) you don't need sensors much. Otherwise, you will need to upgrade those as you run out of good planet target. It's a bit complicated to describe what planets are considered good, talk to your legionmates for a bit more on that. You don't need to up sensors IF you don't plan on scanning planet for a while.

Rank point wise, early on pure energy(20-30 ranks) or split between energy and deck(~50 ranks)
after that u will want to focus on deck until u can more or less hold everything of value (pretty much constantly everything other than atomsphere deci and cloak/scanners at same time, thou u can also consider skipping shields)
and then u wanna go back to energy until u get at least 10x rank in max energy
and after that u can split between tacts and energy

Just make sure that you NEVER put any pt into helmsman, scientist or cargo, and u won't be wasting anything.

This is for regular build. There is also a tiny build where u maintain a deck of rank+19 and split RP between tacts and engineers but I personally don't like that so~

And then there is the mission-auto-lvl tactic of enough deck for energy module only and rest all energy... I personally don't like that much neither but.... the choice is urs

I would say stop investing in decks at Star Destroyer/2k decks (I'm aware Star Destroyer is 2050 but... whatever) and letting shipbots carry you, as opposed to basing it on how much crap you can fit.

Also, later on, I'm always happy to draw up builds for low ranks...

^^^
That would be helpful. I'm forcing my friend to play GL (I'm spending Paddy's Day in his house )

i tried, but most of those who would actually be interested in it are busy, and rest arent that much in math to play this... sadly

My friend is a huge gamer, is pretty bright so he'd understand the many intricities of GL, and he has only one hobby: swimming at half 5 in the morning before school.