If it's still on sale, buy it. It's what you want, and a good price for it.
On video modes: actually, with the avga you only select resolutions, as the card will automatically set refresh. (You could also just use Soft-15khz for free...and AdvanceMame is also an option, but DOS experience is required to get it running right.) Another thing is, not all vertical (or horizontal) games are the same resolution, so you will have to find one for each that will fill the screen - or as much as possible and then using the H and V controls in the OSD to size the image to the screen.
(related) On orientation: if you keep your monitor in horizontal orientation, then of course you can only use so much of the screen for vertical games if you want them to look like they originally did - thin and tall. Now, let's say you set Pac-Man to 'auto' for resolution, and 'default' for orientation. Mame happens to be able to find the right resolution that it fits right, top to bottom. Same if you change the orientation to 'vertical'. It automatically sizes it just right. But some games (again, thinking horizontal orientation with vertical games) require you to find a resolution cos Mame doesn't select the right one. You will then size it via the OSD controls. However, again, if you change the orientation to vertical, the monitor settings have not changed. So you have to resize the image. In some cases, the resolution may not give you enough to resize in vertical orientation, so you have to find another one for that, then resize if needed.
Got it?