Stew Mac is not the place for finishing hardware. I buy my sealer from them when I run out (high prices). They have a lot invested in selling lacquer to amateurs, and Erlewine is their mouthpiece, so you won't be getting any state-of-the-art technical knowledge or materials or equipment from them in this lifetime.
I like the Behlen Sealer. RIC uses thinned down conversion varnish as sealer and it bonds even better than Behlen. All sealers must be scuffed BTW.
Lacquer: I use DuPont 480S, which is not a nitro, but an acrylic clear. Good stuff and can be tinted with acrylic or urethane tinting colors.
For a good gun at a decent price, try the Sharpe Finex FX300. Perfect gun for guitar-sized jobs. The small gun in your two-gun kit is too small, the big one too big.
I recently refinned a bass with a new Sharpe Finex FX300 and the gun sprays very, very well with great atomization and control. It has a nice feel, too, which most Chinese guns lack entirely. You can find these online for $75.00-100.00. It's an HVLP, too.
I use a separate gun for metallics and pearls, another for solid colors, and a third for conversion varnish. My sealer is in a fourth gun! Each a different gun, with no possibility for cross-contamination.
Here's my gun rack:
Left to right, new Sharpe Finex FX300 (solid colors), Binks MG-1 with 3M quick change disposable cup system (clearcoat), SATA Minijet (burst shading), Binks MG-1 #2 (spare), ATD (Taiwanese)(sealer), UPOL (Taiwanese)(metallics), and ATD #2 (spare). The purple Binks guns and the green SATA are all pricey professional guns. All are five years or more old and bulletproof. The Sharpe is only a few days old, was recently engineered after deal problems, and sprays like a $400.00 gun.
Taiwanese guns are getting pretty decent and are good value for the buck. The pro guns are between $300 and $400 each, but last forever.
Here's a 3-pack of Chinese guns selling for about $120.00 on the net:
I purchased these to evaluate. They are not very good. But knowing the Chinese, give 'em about two or three years and they'll get it right. These guns feel bad (not smooth to operate, and the machining on the nozzles is bad).

“I say in speeches that a plausible mission of artists is to make people appreciate being alive at least a little bit. I am then asked if I know of any artists who pulled that off. I reply, 'The Beatles did.”
― Kurt Vonnegut