... I plan on reviving the tank and making it into a small saltwater tank, ... I would probably like a Pleco and some cool looking mid dwelling fish and that sort of stuff.
This form is primarily for planted tanks, and are almost always fresh water systems. Every one here will try to help, but it's often not their area of expertise. You might be better off posting your questions on a SW forum. There are a lot out there.
Reef Central is the largest, but has the reputation for sometimes beating up on people new to the hobby. You can see them here - Reef Central Online Community
I tend to post on Reef Sanctyary which is a lot more beginner oriented you can see them here - Home | Reef Sanctuary
While there is a lot of good information on the net, going through it is quite a task if your new to SW systems. I recommend that you get yourself a couple of books and read and study them. Here are two to start with -
The Conscientious Marine Aquarist
by Robert Fenner
The New Marine Aquarium
by Michael Paleta <---This book has an especially good section on fish suited to someone starting off in the hobby.
A note on the books. You can often find used copies at much lower prices at places like Amazon. In most cases these are going to be as good as new copies, especially when your first learning.
This is information that you can not easily obtain from the net. While it's out there, it's all over the place, and there is a massive volume of information. There is also a lot of bad information out there.
Before you purchase anything, consider first what you plan to keep in the tank. That can drive the equipment selection.
You are going to find that while the goals are similar, SW systems do things very differently from what is done in a FW system.
As a note, Plecos are a FW fish, and can not survive in a SW system.
20 Gallons? 150W to be on the safe side. It should only be a couple bucks more than a 100W. Reason being is it won't have to be on as often as a 100W would be, therefore giving you more life out of it. If you have kept a 5 Gal previously I'm sure a 20 will be ok for u. I recommend dry rock for sure, if you can get fiji rock where you are. Patience is always a huge help with saltwater, if you use dry rock you don't run the risk of introducing any unwanted pests or parasites into your tank. But to be honest a full cycle will take close to 2 months with dry rock.
Heaters are something you don't want to oversize. I would consider a 150w heater on a 20 gal tank to be a mistake. It's really far too large. This causes it to be somewhat less effective. A larger heater also makes it a lot easier to cook you fish if it fails in the on position. It's kind of like installing a super large furnace on your house. It will go on and off a lot more, costing you more long term.
I would recommend about a 50w heater for a 20 gal tank. in other words, about 2 to 3 watts per gallon. If you keep the tank in a place like an unheated basement then maybe a 100w heater.