I don't think a Glock 35 with a conversion barrel is Production legal. You are unlikely to be hassled on this at a level 1 club match, but the gear rules are the same at locals and majors... only intensity/rigor of enforcement is likely to differ.
waktasz has done very well in production shooting 40 minor. As you know, that can be made comically soft-shooting... the greater expense of 40 projectiles seems to be the only real drawback for the person who makes their own ammo. Heck, you could drop down to 165's at that same velocity and still hit minor PF with ease.
Limited is a high-cap division. Absolutely everyone will have mag extensions up to the legal (141mm) limit. Nobody who is remotely serious about the game will not have 19-21 round magazines in limited (assuming major - they'll be up to 23 or so if they're shooting minor). Running a 15(?) round magazine won't absolutely kill you, but you'll find it annoying and it will be a distraction to you. If you want to shoot limited (the division I've spent about 99% of the last 5 years shooting), go ahead and gear up.
Number of mags: In limited, my mags hold either 19 or 20. There are some 30-round stages where there's not a good reloading point right in the middle, so every once in a while I end up with 2 planned mag changes (especially if there are targets that I feel are likely to warrant having make-up shots on board for). So that's a 3 mag minimum. And every once in a while, you will either drop a mag during a reload or get a jam that is best cleared by dropping the mag and putting in a new one. So that's 4 mags. Four also gives you a little bit of freedom in a multi-string stage with mandatory reloads to avoid digging dropped mags out of the dirt and immediately throwing them back in your gun. I have gone to the line a number of times with just 3 mags, but 4 is my standard compliment. Many people make do with 3, though.
For similar reasons, some people run 5 mags in production, but I would want 6 if that were my real division. Production involves throwing a lot of mags on the ground, and most of the time they will still have rounds in them. Having 4 planned reloads on a stage is common, and 5 is not unheard of. Throw in a jam or a dropped mag, and things go sideways if you've only got "just enough" mags for your plan.
So, to sum up on number of magazines: Figure out the largest number of magazines you think you will ever want or need on a stage... and then carry one more than that number.
Except for the very first part, this is all just my opinion/reasoning.
