when you say combo, are you referring to one stock & action that you can have a separate set of 20 and 12ga barrels for? Is this for skeet or something where you want multiple gauges, or is this for versatility when hunting?
That's an awful lot to ask for $800, making a good set of double barrels isnt cheap to begin with, and then you have to fit 2 sets of barrels, time 2 sets of ejectors, etc. I would not recommend that gun--have seen several shoot themselves loose within a short period of time of shooting targets with them, it's not a gun that will last with any heavier use.
Not sure what your intended use is, but from spending time shooting clays I have yet to see one of the lower-priced doubles last very long when shooting any kind of volume--the constant opening/closing causes the action fit and locking lugs to wear down prematurely and within a year the gun is loose beyond repair. Even if you are hunting and not putting a ton of rounds/year through it, it still equates to a gun that wont last as long as one with a better fit and replaceable locking parts. This is why most folks will recommend either a better quality double gun, or a pump or auto--in the long run you get more gun for your money that way.
Unless you need the full different set of tubes for a competitive format (in which case see above volume shooting comment) if you want an O/U I would stretch and find a a used browning or beretta--whichever fits you better--and use ammo to stretch the lighter/heavier capacity of the gun. An older 12ga base-model like a beretta 686 essential or even an older BL3 or BL4 or similar are actually within your budget usually and will be a far better quality gun, even well-used. You'll get a better, longer-lasting gun that will hold its value that way for not that much more than what you would have spent. RST and a few other companies make 7/8oz 12ga loads for example that are great for partridge or quail, and the AA or STS 1oz target loads are also fantastic bird hunting loads as well as for clays--either one is the equivalent of a typical 20ga field load. You can do the same with a pump, but many 12ga autos wont cycle lighter loads so that may not provide the same versatility.
Another thought with an O/U combo gauge set--in nearly all cases you will generally get the action-size of the larger gauge, i.e. you are getting a 12ga action even though you are putting 20-ga barrels on it sometimes. In most cases this means you are still carrying around the full weight of a 12ga gun for the most part (In some cases the 20ga barrel set is actually heavier than the 12ga!), so the only thing that really changes is the color of the shells you are putting in it. To me, this is a good reason to forego the second set of barrels and simply just adjust ammo as needed.