You are absolutely correct, the ion can work on both hp and lp and adjustable, some markers such as angels can have problems with hp but IONs run on it fine, in fact personally I would recommend it
This is for a number of reasons, the two main ones being you will have less shoot down with the hp reg, and hp bottles can be used on most guns other than some such as angels but not all guns can run on lp (eg mechs cockers)
The best bottle I can recommend is a 5000 psi, 1.1 litre hp for an ION, 5000 psi is better imo just in case you get a flash fill or the air heats up increasing the pressure above the 4500 you can fill it to, a 1.1 is also better for me personally since they can hold a lot more than a 0.8 and are about the right size for most people if used with a normal rail or mini drop. The other main thing is you should make sure you get a fibre wrapped bottle since these are a lot lighter than the steel alternatives
Cross fire or pmi tanks will both serve you will, as I said I have a 1.1 litre 5000psi and hp, or an decent adjustable reg such as a max flow or angel air, but these do cost more
1.1 5000 psi bottles do cost a lot more, but they do have advantages over the steel 0.8 3000 psi
Simple maths will prove the amount of extra shots, just remember a couple of points, with a hp reg once the pressure inside the bottle begins to get lower than the output of the reg, the bottle will no longer be able to supply the correct pressure to get the velocity you want, and as most hp regs are 850 psi output once they reach below this you will notice slow recharge rates and velocity problems, so the last 850psi is could be counted as useless in most situations
3000 – 850 = 2150 psi, which is ‘good’ or useful
2150 x 0,8 = 1720
4500 – 850 = 3650
3650 x 1.1 = 4015
4015 / 1720 = 2.33
2.33 x 100 = 233%
and as a ratio the 1.1 5000psi tank can hold up to 2.3 times more air that will allow the ion to shoot properly than a 0.9 steel 3000psi bottle
meaning you only have to refill your air less than ½ the time and you can take out twice as much paint for big games and walkons
they are also a lot lighter and eventually you will be upgrading your marker (if you stay in the sport which hopefully you should) you will be wanting that 1.1 eventually to get the most out of your guns
If there is anything else I could help you with feel free to ask me
-Mat,