It is not worth it. At 750-1000 points (and I would argue at any point level) upgrading sarges with plasma pistols is a colossal waste of points. Power weapons MAYBE but only if you are going to be getting into melee. Honestly, for tactical squad sarges I would give them nothing or a combi-weapon matching the special weapon (so a combi-melta if you use a melta gun for example). Melta bombs is a good cheap option too.
For devastators, I would equip them with weapons that can take on similar targets. Lascannons are always a good call for anti-armor. Missile launchers give you some options. I would avoid multi-meltas; they just don't have enough range.
Overall, I would avoid kitting units out too much. You don't have a lot of points to work with.
Here is a sample demi-company at 1000points that works with Ultramarines:
-Chaplain w/jump pack, melta bomb 115
-5-man Tactical Squad w/melta,combimelta in rhino w/dozer 130
-10-man Tactical Squad w/flamer, missile launcher in rhino w/dozer 200 (combat squad would work well)
-5-man Tactical Squad w/plasma, combiplasma in rhino w/dozer 135
-Venerable Dreadnought w/twinlinked lascannon, missile launcher 150
-5 assault marines w/jump packs, power weapon for sarge 105
-6 devastators w/4 lascannons 154
It's pretty basic but you cover all the bases nicely: you have tacticals to grab objectives, you can take on most types of targets, you get to take full advantage of all three doctrines for the Ultramarines chapter tactics.