Ok, sjad, you have good questions. This is "Reloading 201, Beyond the Basics" stuff.
First, that "bulge" above the belt; people who have a minimum tolerance chamber and a max tolerance sizer may have difficulty chambering rounds that have been reloaded several times because that bulge becomes swells too large. If that's the case, the fancy collet body sizer will help but so will a less expensive modified FL sizer, as Buffalobob describes. More info on this in the closing note below.
Understand that if your rifle chamber and the die fit is proper, neither the $80 collet die nor a modified sizer is needed.
Second, to get your headspace right, ignore both the belt AND the conventional "touch the shell holder with the bottom of the die" instructions for adjustment. For best case fit and longer case life, you need to adjust the die down only far enough to set the shoulder back 2 or 3 thousanths.
To do that, first adjust the die down conventionally and then back it OUT a quarter turn (that being about .018" high). Then fully size a fired case and try to chamber it; it shouldn't go easily, if at all. Now, turn the die down in tiny 1/32th turn steps (NO MORE!), that being about 2 1/4 thousants per step, and repeat the test fit. It probably still won't go at first. Keep this up until you can close the bolt with little or no resistance, your choice, and go no further.
Lock the die ring so you don't have to repeat the adjustment and do the rest of your loading normally. And always use this method to adjust any FL sizer for any bottle neck case; belted, rimmed or rimless.
Using an RCBS Precision Case Mic, Stoney Point or some other tool to measure the head to shoulder length of fired and sized cases is somewhat easier but no more accurate than using your rifle's chamber as a gage for your sizing progression.
** Personal Notes on modifying a FL sizer for belted case bulges:
I don't think you will be able to cut any case hardened sizer with a conventional hacksaw. But, I have drilled out the forward part of a die with an inexpensive (Walmart) 1/2" carbide masonary type drill bit.
I found a .300 magnum FL sizer for $2 at a garge sale and took it home. Took out the top guts and clamped it upside down in my drill press vise and drilled clear through the die, entering from the bottom. That removed a short section of the body and ALL of the shoulder/neck portion. Then I chucked up a short section of 3/8" dowel with 400 grit black sand paper snuggly wrapped around it to smooth the drilled section.
Now I use the modified die to size down the dreaded case bulge on most belted mag cases easily without moving the shoulder at all, no expensive collet die needed!
Well, it almost worked. On one set of cases I still needed to grind off just a few thousants of the bottom (maybe .003"?). After that it worked fine to restore normal case diameter just above the belt on everything I've tried it on.
I see other FL sizer dies standing alone on ebay from time to time> Some are too expensive for this mod, some are not. But it's fun to do things different, still get the job done right and save money too!