It looks like an RS80 and I just finished rehabbing one myself. This skimmer is the best for a mature mixed reef tank up to 80 gallons, or a heavy FISH ONLY tank up to 50 gallons, or an SPS heavy tank up to 50 gallons. You could use it on a 100 gallon tank but you are going to have to watch your bio load as the tank matures. Those are the manufactures "Real World" recommendations, compared to some skimmers the RS80 would be rated at 250 gallons.
Measure across the top of the collection cup and let me know the diameter, as well as the total height of the skimmer and I can advise better of the model. Jeff at ReefDynamics can provide even more accurate information than I can. He designs and build these things, I only own a RS80 I refurbished recently.
From what I see you are missing the pump, input assembly and uni-seal.
You probably will need to replace and lube the silicone O ring for the Jam-Lok, and possibly the airline (if it is brittle or too short).
You will want replace the output manifold with a wedge pipe outlet assembly, as the one on there is a real pain to use.
ReefDynamics sells all of the replacement parts for the old EuroReef skimmers, and Jeff is great about advising the exact parts you need.
Keep in mind that if you get all of the direct replacement parts this is going to run you about $215. Some of the replacement parts are better than the ones the skimmer originally came with, as there were improvements over time from what you have. While I do not have experience to compare possible generic parts, I have seen the reviews when Euro-Reef came out with several of the improvements, and it made a large improvement to people the upgraded at the time. So in my estimation it is worth it, and if you are going to use this on a 100 gallon you will want every bit of performance you can get.
When you are done you will have a skimmer that retails brand new for $300.00. If you plan on really filling out your 100 gallon tank, and growing it out for 3+ years you many want to consider picking up there INS135 instead. The INS135 goes for about $400.00 and has a "Real World" rating of 110 - 135 gallons, and comparatively up to 400 gallons.
The tanks that I would choose for a sump off the top of my head in order of preference are 40B, 29, and 20L. Generally you want to go as big as you can while still leaving some room under your tank for other things such as top off water, dosing equipment, etc... A 40 gallon breeder would be a nice sizable sump, with plenty of room for filtration, skimmer, return, and still have room for a fuge or other gear, but you could go as small as a 20 Long and still fit the basics and a RS80 skimmer.
