Well Proto or a Custom Product on/off's are good ASA's and are relatively cheap, take a look a some of them.
In regards to bolts, you should look at the fire bolt or the TechT L7 bolt. Both are good bolts but I prefer the L7, it's smaller and lighter then the Firebolt and it operates at a slightly lower pressure then the Firebolt. But it's upto you which you get, both are really quite good.
You'll also want to get a QEV, that helps the Ion quite a bit. It helps to improve efficiency and helps improve the cycling rate. It's definitely a must buy.
When it comes to boards for the Ion, there are a few to choose from. There is the Blackheart Board, the Virtue Board, the Spitfire Board and the Ape Board. They're the one's I'm aware of and have found on UK online stores. The Blackheart Board is the cheapest, but it's also a bit of a pain to adjust whereas the others are better to adjust. However, I don't have experience with the other boards, but from what I've seen and read they're of similar abilities.
And if you are looking for a better barrel, then a Un1tec barrel is a wise choice.