The Eheim is an excellent filter and would be plenty for a 55. The thing is...you don't need it...you don't even want it.
SW is different than
FW in that the
LR handles all the biological filtration. The only mechanical filtration you need on a reef tank or a
FOWLR is a good skimmer. Take the money you were going to spend on the Eheim and buy a better skimmer. Have you thought about setting up a sump? You could hide the heaters and the skimmer in there. There is also a better selection of skimmers available for in sump use.
You'll need about 80-90lbs of
LR for a 55. You may also want to look into using a
DSB. There are arguments for and against these...I use this method and feel that it is a viable way to control nitrate and add biodiversity to the tank.
JMHO though.
The reason you don't want the canister filter is that it will churn out nitrates. Nitrates are something you don't want in a
SW tank and you REALLY don't want in a reef tank. Many inverts are very sensitive to nitrate. The
LR and
DSB will not only control ammonia/nitrite, but will keep nitrates in check as well.
One other thing you'll want that I didn't see mentioned in your post is powerheads. You'll want a minimum of 550gph flow in a 55g tank. More is better. I would recommend Maxi Jet
PH's...they are cheap, powerful, and dependable.
If you decide to go with the HOB skimmer, especially if you intend to upgrade to a larger tank later, I would recommend the Berlin HOB or the Aqua C Remora Pro. I would not recommend the Prizm Pro...been there...didn't like it.
As far as how many fish you can stock, you really need to decide what you'd like to keep and then we can see if they will go in a 55. For that size tank, I'd stick with smaller fish...no large angels, ect... Remember the lighter you keep the bioload, the easier maintenance will be.