>specific books for tech level
Yes, always.
Since you're at a transition point, get GURPS Low Tech AND GURPS High Tech.
Low Tech has muskets & related things, High Tech has rifles and some more gun related rules.
You MIGHT also want GURPS Martial arts, but that could be a bit much. You do want How to be a GURPS GM though.
Related to extremely rare magic, I recommend using GURPS Ritual Path Magic. It's really good, but you will need to really get a handle on it. Once you do, you won't regret it. It's definitely regarded as the best GURPS Magic system. (GURPS has multiple magic systems you can choose). Ritual path magic definitely has its own flavor though.
Spells are cast rather slowly, but they can be 'stored' in items (as charms), or 'hung' on people, to trigger conditionally.
So a wizard doesn't walks into battle with wands and charms, flinging explosive beads, cracking a necklace to stun his opponents. These are things he would have prepared earlier, though RPM mages do have a small mana pool to draw from in the middle of battle.
It's also great because a character can be a mage without having a huge point investment. They WILL need to go full cultist-mode with magic diagrams on the floor, incense candles and a piece of a victim's hair and all that though.
http://www.sjgames.com/gurps/books/ritualpathmagic/
There's a lot of support on the SJ games forums for it, and even some examples of people making their own magic paths.
For details on other magic systems in GURPS, & a general comparison: http://pastebin.com/4Wk6gB2D
>Advice and howtos:
Use the semi-official errata: Will & perception are not based on IQ. Do it.
2nd, USE TEMPLATES. Your players are going to be swamped in all the options, Templates cut through that by only providing the options that are relevant to them.
I'm not really sure what to point you to for templates, since it could vary wildly depending on what your players are doing. I usually make my own templates for my players, but I've got a decent chunk of experience so I feel comfortable doing so.
At the very least you'll want to look at Kromm's stuff. Kromm one of the GURPS staff and often posts on the forums (the GURPS crew are very communicative).
You WANT http://gurpscharactersheet.com Included under classes > kromm are templates assembled from his words of wisdom; action hero, adventurer, and everyman, with instructions like "pick one of X." You'll want to start each character by running them through those
The everyman template has a few things that add flavour to a character, but are NOT useless.
The action hero template includes:
>picking a grappling skill
>picking a melee skill
>the holdout skill (aka hide-items-on-your-person, the skill)
>a ranged weapon skill
>an unarmed skill
>throwing skill
These are good and pretty much every actiony hero needs once of each of those skills.
The Adventurer template expands on this, adding perception skills for finding and discovering, outdoor skills for getting around and getting to places, and choosing people skills for interacting with "your people." If your characters are doing adventure things,t hey will need to use all those skills at one point or another.
Finally, the Everyman template is worth looking at for some flavour. Don't forget area knowledge, every character should have it unless they would have it for an area never visited in the campaign.
Finally, here's something some GMs (to my surprise) forget:
WHEN A CHARACTER IS NOT IN COMBAT OR RISKY SITUATION, THEY GET A +4 BONUS TO MOST SKILLS. THAT IS WHY A SKILL OF 10 IS CONSIDERED ACCEPTABLE IN MOST THINGS.
A +4 bonus is a lot in GURPS, and this makes sense. For example, there are people who can get great groupings down at the range, but in a combat situation their accuracy is shit. (This is true of most soldiers too). The difference between doing something in a normal environment, and in a combat environment is enormous.
This is also why a skill level of 14 is considered "expert," as outside of combat they have an effective skill of 18 and thus a less than 1% chance of failure.
Lastly, don't make your players roll for random shit just so they roll. I've played and run GURPS sessions without a single dice roll because there was no situation that required one.