Can I just, take everything back a step?
You initially listed a setup, which is what people tend to do when they don't know much, which is fine.
Are you intending to buy this new?
What is your budget?
Reason being...markers lose money...quickly.
Generally, the only wearing parts on markers, are o-rings, valve seats etc, which are all easily replaceable, so even a well used marker, can be like new mileage wise, very easily.
So...if you have some sort of budget you can post up, then we may be able to help you select a better setup, from the secondhand market.
If you are one of those people, who MUST have brand new...that's fine, get on with it.
But if like most of us, you don't have piles of money to waste...we may be able to help.
Proto Rail, is a slightly better marker, than the Ion too.
As for Crossfire LP HPA. Crossfire regs are extremely fast and accurate flowing regs, which is why many, usually HP markers, will run OK on a Crossfire LP reg...especially seeing as the Ion's inline reg, is also extremely good. But, as has been said, you may experience drop off in the higher BPS region.