Hello! Welcome to the Tau!
A couple tips overall:
1) Bonding Knives are *never* worth it. Never pay for them.
2) Team Leaders and Shas'vre are almost always a waste of points, so skip them (there are some rare exceptions to this, but generally).
3) Markerlights are very useful, but you don't *need* them to make Tau work, especially with the formation rules available.
4) It is really, really easy to pitch away a lot of points on upgrades that you don't especially need, as most Tau units are pretty solid with their basic load-outs. Given that Tau units are super cost-effective to begin with, you could sometimes ditch a lot of random stuff and buy an whole new unit or two, which is better.
5) The Hunter Contingent is incredibly good, and you should aim to build it at your earliest convenience (point level permitting--I'd say 1250 point matches and higher it is totally doable).
Okay, with that said, about what you're thinking specifically:
Fire Warriors w/Pulse Rifles
This is my preferred load-out--the pulse rifle is flexible, long ranged, and generally useful. If you know you're going to be putting FWs in Devilfish (which I also love), giving them Carbines may actually pay off better. Personally, I prefer the flexibility of having that long range when needed, and I rarely miss the extra 3" of double-tap range the pulse carbine grants and pinning is almost never useful.
I don't especially like Breacher Teams, mostly because their optimal range is so short as to make them a suicidal unit (unless you're blasting away a small unit--like bikes or something--and there's nobody else there to counter-assault). They're a little awkward to use. Strike teams are more useful, I think. Then again, if you're dealing with Ravenwing bikers with crazy re-rollable jink saves, Breachers might seem more appealing.
Don't buy the fusion blaster. Just don't. You've got a unit that's a great anti-infantry unit that can also blow the hell out of AV10 and scare AV11 and you're cutting that firepower by a third to buy a single fusion blaster that's going to miss every time it's important, anyway. Stealths are not anti-tank units, and trying to make them so is counterproductive.
As for the markerlight or homing beacon on the Shas'vre, consider the points you're laying out and the benefit you're getting from them. Unless you plan on deepstriking a lot of Crisis Suits (Retaliation Cadre, for instance), I don't think the Homing Beacon is worth its cost *and* the cost of buying the Shas'vre in the first place.
The Markerlight and Target Lock are a slightly different situation--if you're low on Markerlights and need an extra, it's fairly cheap and also on a Relentless platform, meaning it can move around and still fire at full effectivness (hard to get with Markerlights). Depends on the rest of your army, though.
Pathfinders are definitely the cheapest way to put Markerlights on the board. They are also the most fragile markerlights you can get. In my experience, once an opponent gets a taste of what markerlights can do, they will plan on killing your pathfinder teams ASAP. And they can, most of the time. I basically stopped using pathfinders because the damned things never lived to see Turn 3 and the Coordinated Firepower rules (from the Hunter Contingent) made them redundant.
One thing about markerlights is this: you can take too many of them, but it's hard to take too few. Too many and you suffer from markerlight overkill. You rarely need more than 3 or 4 markerlight hits on the same target (fewer with Coordinated Firepower). You also rarely will be able to effectively focus firepower on more than 2 enemy units a turn, and spreading out your shots more than that is counter productive. Accordingly, I don't take more than 6-8 markerlights spread across 2-3 units in a 1500-1750 point army. It's all I really need, and any more are taking away guns that *actually kill* the bad guys rather than simply target them. Even 3-4 markerlights is still perfectly useful. 12-16 is mostly a waste.