What gun you should get in what order is up to you, but my advice on shotguns is that you get a 870 Express, or a Mossy 500. There is no need to get an auto loader, or to get a brutally tough police 870, unless you plan on butt stroking rioters and generally abusing the hell out of it. Out of the box, with a bead sight and a barrel of 21" or less, the pump action 12 gauge is the most formidable small arm available to us without extra ATFE paperwork. Getting a used one is the route I went, and the route I suggest you take. I got mine for 200 bucks.
If you want cheap ammo, you need to reload it. And even that is getting harder. Getting a gun for ammo costs, other than a .22, is probably a mistake, unless you are getting a LOT of ammo right up front.
On the topic of what would be best for HD, I think it depends most on how your house is constructed. There is a huge difference between brick walls and wood walls when it comes to penetration. The very first priority should be not risking hurting others in the process of defending your house.