I'm no expert, just a guy who has a bunch of mostly smaller black powder cannon. Most of what I shoot is mainly 1" diameter bore and smaller, they are way easier to clean since you can do them in the kitchen sink. I also have a half scale Napoleon and a Lyle gun and both weigh right around 160-180 lbs, you shoot them once you have to clean them the same as if you had shot them a hundred times, the bigger ones also eat more powder.
For someone getting started I recommend looking for a mortar, preferably golf ball or beer can sized, ammo is cheap and plentiful and you don't need much range but you still get all the smoke and noise.
I've been two years building a field carriage for my Napoleon and it still isnt done, mainly because it sits in a workable but nowhere near correct naval style carriage now. Barrels aren't expensive compared to once you start looking for someone else to build a correct carriage, wheels and woodwork don't come cheap. If you can do the wood you are way ahead in the game. And for the record I'd also love to own a full scale mountain howitzer.