Material: You will have either a choice of glass or acrylic tanks, for tanks in the 200 gal and below range I would prefer glass, it is more durable, not as easily scratched. The only benefit I see to acrylic is in its lesser weight and higher strength than glass for truly large tanks. They are of course easier to drill for fittings also. With being a softer material than glass, you must take care of it properly and be aware of what cleaning equipment is used on it since you could end up with deep scratches or even a fuzzy haze appearance due to multitudes of small scratches. For those who are do it yourself buffs, I feel glass is also easier to work with and bond. Also keep in mind, for those truly adventurous types, you can also make a tank out of plywood and one piece of glass for the front panel.
Size: Of course the size you pick will also determine the overall cost/expense of the needed equipment for it such as lights and filtration, it will also determine what species of life you can keep within it. It has been mine and others experience that it is best to just go ahead and get the largest tank you can manage right up front. When you get that 40 gallon tank and start to stock it, its only a matter of months before you will be drooling over that 150 gallon tank you saw at the LFS and wishing you had gotten a bigger one to start with. For me, anything below a 75 gallon tank is just far to restrictive,unless of course you are planning a species specific tank such as for seahorses, or just wish a little 10 gallon nano-reef tank for your desk. But for a show tank, go as big as you can right away and just get it over with.
Predrilled or not: This is where you will have to really plan out your reef's design, the most common reason for having a tank drilled is to provide the overflow needed for a sump. Which is a great way to provide a place for all that unsightly equipment such as skimmers and heaters out of sight, it also provides a bit more water volume and makes doing water changes/supplementations easier. The big concern is where to drill the tank, on the back,the side or through the bottom? Where the tank and sump is going to be situated will play a big part in where to drill the tank. Keep in mind that drilling is not mandatory, there are other ways to perform the drain function such as with hang on overflow boxes. As with everything within this hobby, planning, researching and alot of heavy thinking will go a long way in making your tank set up alot less frustrating and stands a better chance of success and enjoyment by you.