Eat lots, and lift heavy. If you don't have weights, try one end of your couch.
I've been fat my whole life, and I'll tell you- once you're fat, the leg muscles develop themselves. Just keep walking.
Eating lots of proteins, carbs, and sugars will get you bulked up.

Based off your profile height, what you do might work.
But, at 6'3" or whatever I am, I find that most shirts don't go down far enough for my gut. So if you get larger, you might find you need to put your belly in your pants.

If you continue getting bigger, you're going to want to consider the rise of your jeans. They'll need to be deeper in order to hold your gut in.
As for budget options, I don't know what all is possible in ON, but try Penney's clearance, or even thrift or consignment places.

I have a big gut, and no ass, but I don't ever have this problem... I use a thick, wide belt and it holds up my pants fine.
Maybe it's a question of belly fat density. My gut is pretty solid.

Not at all your height, size, or even gender, but I'll just say you can use mybodygallery.com.
Pick your basic body shape and height, then increase the weight.

The rolls under my arm getting thick enough that my arms rest at a wider angle. And then those rolls joining on my back. And then those rolls growing another roll beneath them...

I typically have to make that call myself. But, I was stopped from sitting in one of those flimsy metal lawn chairs once. Not only was I too heavy, but probably too wide. I would've gotten stuck.

I know I've said it on here before, but I managed to warp an army bed. I actually did damage to every bed I had in college through sheer weight.

I also get these Fiji visits. Almost half of my notifs are Fijis visiting my profile.

For me, I would guess somewhere in the 500s is where I'd have to be sitting most of the time. 600s for not doing housework, even occasionally. So I'd say somewhere in there.