FWIW, a Scotch Ale is not a very good first brew, IMHO--it's big, takes a long time to taste it's best, and because the hops element is quite minimal, any little flaw is going to be obvious. (Or, put another way, higher hops leves can mask small "defects.")
I would say, get started right away on that porter, and give the big beer plenty of time.
My advice would be to purchase a kit from Morebeer.com, Austin Homebrew, or Northern Brewer--those guys do huge volume business, and the LME you get from them will be fresh and won't be in a can. Or use a recipe calling for all DME.