I currently use a vamo 5 ecig and would like to upgrade
Finding it very difficult with so many brands and mods to choose from
Any opinions on the best way to research the best mods for your price range?
The first consideration is fixed tube or telescoping. The latter has lost ground in popularity. Fixed tube mods can come with three dedicated battery tubes, or if the shorter batteries are less likely to be used, just an 18650.
Contact materials are important, especially if you're going to run deep sub-ohm. Most mods will do 0.5Ω and higher without issue. A low resistance/high conductivity mod is required as you go deeper.
Copper is the material of choice... and higher end mods can have silver, rhodium, iridium or palladium platings to reduce corrosion and/or increase conductivity.
Adjustability is important if you want all your attys to sit flush, as well as your cap to battery tube joints. A dual telescopic 510 post is common, and sometimes even and single telescopic negative post can be found. My favorite is a floating 510, with a threaded positive battery contact.
Speaking of bottom buttons only... buttons can extend from the base surface, flush, or they can be countersunk below the lockring (if any) or the switch body. If a lockring is used, left hand thread is preferred.
Magnets and springs are the button counter-force. If done right, magnets work well, if done wrong and require very little or too much pressure... you're kinda stuck unless you can find different magnets of the same dimensions. This is where springs shine, in that the pressure is easily tunable.
My personal preferences are stainless steel or hardened aluminum fixed tube, absolute minimum firing pressure and button travel. A flush or slightly countersunk button, no lock ring... or if a lock ring, the button is still countersunk slightly into it, so that if not locked, the mod can still stand on end without firing.
Why stainless? Copper and brass are appealing, and some say that these materials improve conductivity... but that's not really true, considering there are zero voltage drop, stainless mods. I know, I own a few.
If you like the look of copper or brass, that'd be the main reason to buy one... but be prepared, especially with copper, to have thread wear and tube dent issues.
Along with the posts above mine, you have a good place to start.