Welcome to the forum.
Based on your maximum price I would suggest that for now you stick with a factory made gun. Preferably one like a Cabala's or Lyman or a used TC.
I know you want something that you won't want to replace but if you stick to one of the rifles I mentioned they will hold their resale value pretty well. That will allow you to save for one of the higher priced guns while you learn about muzzleloading with the lower cost guns.
Then when you decide to move up you will be able to sell your first gun to help pay for the new one.
You didn't say where your from so I need to point out something.
A Flintlock gun needs to have real black powder to prime the pan and to load the main charge.
In some areas of the country finding real black powder is darn near impossible.
Because of this, I recommend that you first call around to the various gun stores in your area and ask them if they carry real black powder.
If they do, your in business. If they don't your only option will be to mail order it.
Black powder has a Haz Mat charge that can run over $20 per shipment in addition to the price of the powder so that makes for some high priced shooting.
A Percussion lock gun which uses a copper percussion cap to fire the powder will work with the synthetic powders as well as black powder. Just something to think about.
As you mentioned deer hunting, your caliber choice should be aimed at a .45 or larger bore. The .50 and .54 are more than enough for deer and the .50 is probably the easiest to find ammunition for.
If you like the idea of building your own, that's another option but remember, the "kits" from Track of the Wolf, Pecatonica River, Muzzleloads' Builders Supply,TVM and others are not "fall together" kits. They are a box of semi finished parts and can easily take a builder over 150 hours to assemble and finish.
The Kits from TC, Lyman and Traditions are basically factory built production guns left unfinished and can be built in 20-40 hours, depending on your experience and any modifications you might want to make before applying the final finish.
If your interested in this please look over our "Gun Builders Bench" section. It was designed to help new builders finish their projects and no question is too dumb to ask. :thumbsup: