Like you, my budget fenced me in when it came to components. My two-cents.
I use one Mamba Max Pro ESC and it has performed flawlessly. Cheaper than two FXRs and arguably more durable than the FXRs, based on more than a handful of postings on this site.
I still run the original plastic c-hubs and knuckles and have had zero regrets about it.
I tried Holmes Hobbies 55T Sport motors (I think that's what they're called). They're tanks and more than do what I could ask of them and they're very reasonably priced. But I like crawling slow. I also enjoy the 70 min run time I get out of a 1050 LiFe pack. I don't need a trunk of batteries to get my fill of crawling. My XR can't leap a 3' wide gap like a frog, but it'll ease out of nasty crevices and insane binds without destroying the trannies.
Tires. I still run Chisels sometimes because they're a great tire for the rocks here in my area. Even if they're not the latest, they are still a good all-around tire and I doubt you'll find a tire that will take longer to wear down. I prefer Leopards, but since they're discontinued, they're getting tough to find. I'm one of the few jokers who isn't a fan of Rovers, at least for the stuff I play on here.
I never increased bearing sizes in the trannies and have had zero problems. I ditched the bearings from the kit and replaced them with Traxxas or ProTek (amainhobbies.com store brand) rubber-shielded bearings of the same size and have had absolutely no problem whatsoever with any of them.
The only non-stock gears in mine are the final drive gears (one in each axle, where tranni mates to it). These were the only ones I shredded. I replaced them with HB brand, but I would imagine any machined final drive gear will more than suffice. After more than a year of a LOT of abuse, the rest of the stock gears from the kit are perfectly fine. If you jump to a 35T or maybe even a 45T, you may need to swap all the gears out. I don't know.
Again, because of budget, might consider the 6x101 30-degree bent links that Axial released for the AX10 if you want different links. They work great and they're crazy cheap from several mfrs now. Between rod ends for the XR, the AX10, Revo, and Jato, you can come up with nearly any length you want for your setup. I use Crawler Innovations universal delrin lower links for the rear lowers on mine and they've been a great value.
I'm running the same Hitec 7954-SH high voltage servo I had on my AX10 and it has survived nearly three years of constant abuse with no problems at all. Might check it out. Can't beat price on E-Bay.
I cut a piece of polycarbonate sheet (1/16") and fastened that on top of the skid and it gave me room to mount everything neatly and without everything being a nightmare to remove if I need to. I run a 3S LiFe 1050 pack from MaxAmps and there's even room for that (on-edge) between the front upper links on the polycarb tray.
Last thing I can think of is if you don't like the skinnier rear footprint of the XR, a cheap solution is to throw a set of Axial's 8-hole wheels that came stock on the AX10 on the rear and it'll set only a few mm wider than the front. You can often grab a set of these wheels off E-Bay for less than $10.
If you haven't yet found the build tips offered on Axial's website, you might want to check it out. I think it's under the "instruction" tab "XR10." Or maybe they're mirrored here on RCC.
Probably TMI, but hope it helps.
Have fun.
