Joining in on this conversation because I recently started rebuilding a little over a month ago.
I started rebuilding my Protanks at first and once I got good at that I began to realize how better my vape was getting. I'll never go back to factory heads. After about 3 weeks I purchased a Kayfun RBA and a Trident RDA... then it was all downhill for my wallet from there as you can see by the pic below. Learning how to rebuild the equipment I already helped a lot.
There was a learning curve, I practiced everyday for about a week before I was able to build something vapable. Now I can rebuild any of my RBA's in a few minutes. YouTube videos helped a TON. As well as gathering knowledge of all of the mechanics of a APV.
Below is a family photo I took today. It's my vaping journey from when I first started 5 months ago to today in the order in which I got everything. I love my mech mods and I love rebuilding.
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From left to right: V2 cigalike with a Halo Cartomizer, Halo Triton VV with Halo Tank, iTaste MVP 2.0 with Kanger Protank 2, iTaste VV/VW with Kanger Mini Protank, Nemesis clone (18350) with Kayfun/Russian clone, Atmizoo Roller (18350) with Aqua clone, KTS clone (18490) with Trident clone.
TLDR: Rebuilding isn't difficult if you are the type who doesn't mind tinkering, but there is a learning curve. Keep at it and it will be well worth it!