I am building one myself & it's definitely worth buying a set of plans from the sister site smokerplans.com they are very comprehensive & tell you what thickness steel to use, it's a small price to pay to get all the info you need to get started. They hardest division is what size to make & they will answer most of you questions, any others will easily be answered by the knowledgeable folk here.
Ok , the project acquisition is the second step, now I want to know whether to use iron or steel to realize the cost for the material and, therefore , if I may build it.
Ok, now that's a big difference, using stainless on the inside would probably be a great idea, but pretty expensive. Definitely use steel for the frame & fire box, not sure if you can get stainless in 1/4" & 1/2" thickness.
99.7% of all smokers are constructed of steel. (I made that statistic up so you can stop reading here as the rest is worthless)
Carbon steel is easy to work with, relatively cheap and has open pores that take on a nice "seasoning" with use. However, it is prone to rust due to moisture and ash.
Stainless just looks cool. However, it is considerably more expensive, requires skill/equipment to weld and it's smooth finish minimizes pores in the metal. It will, however, last a million years in the middle of a rain forest.
I'm not going to live a million years, don't live in a rain forest and love the season and workability of my carbon steel. Keep a good coat of paint on it, store it out of the weather and you will get a lifetime of use out of it. If you have the resources, stainless is totally cool but I don't see an advantage for me and my climate.