Here's some ideas, and it's just personal preference and things I've learned along the way.
1. Shorter mods look really cool, but battery life and battery safety can be an issue. You may want to stick with 18650 or 22650.
2. I've had fantastic luck with buying clones, and in some cases they are better than the authentic version. With the Nemesis being one notable exception. It's one of my favorites, but I went thru 3 of the clones, all from different manufacturers, and all the switches were terrible. There are a lot of posts regarding getting the switch to work, but do yourself a favor... if you want a Nemesis, just bite the bullet and buy authentic.
3. Telescopic mods are some of my favorites. Not because you can use different battery sizes, but because they will usually accept atomizers with various 510 lengths with no problem.
4. You don't have to spend a lot on a mechanical to get a great vape. One of the first and best mechs I bought was a cronus clone.
5. Fasttech kinda sucks. Their products may be ok, but they have frequent shipping problems and loooong delays. I buy 90% of my vape gear on ebay and have had great luck with excellent pricing.
And that's all I can think of at the moment...