Pretty much agree with Black Ice 72 and Squattingdog.
The SP on/off is the best one for your CO2 tank but is expensive. It can help save your o-rings though.
Almost any antisiphon kit will do. They really aren't hard to install yourself but if you've never done it before it's better to have your local pro sho do it for you. Sometimes if you buy the antisiphon from the pro shop they'll install it for free.
Keep in mind that the antisiphon will only work best in one position. It's best if the proshop has your gun so they can mount it properly for whatever setup you currently have. If you have it mounted elsewhere and it doesn't quite match your setup (normally, the up position on a tank with an antisiphon is marked with an X on the valve) then Lapco sells an adaptor that you can use to orient the tank correctly on any gun. If you're using a remote setup you shouldn't use an antisiphon.
I believe the Imp is made to run on CO2 like the Shocker so you can screw a CO2 tank right on it if you want. The MaxFlo is designed to run on CO2 so you won't damage your gun. There is a chance of your gun getting very cold if you fire fast. Another alternative to an antisiphon is an angled drop which helps to keep the liquid CO2 closer to the bottom of the tank. An antisiphon is a better choice but even that won't keep liquid CO2 out if you tilt you gun in odd positions to shoot.
In general, HPA is a better gas to use than CO2. It's more consistent, not as temperature dependent and cleaner. You will get more shots off a CO2 tank of equivalent size and CO2 tanks are cheaper but sometimes CO2 fills will cost you more than air fills. Screw-in HPA tanks are relatively inexpensive and range from $100-150 for steel and $200-250 for fiber. Top end adjustable HPA tanks can run you around $500 but you can get very nice ones from $350 and up. You'll need to be aware of the hydro date on HPA tanks since they cost so much to buy. Best wishes