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Please excuse the noob question - first Pi project.
Basic project components:
I am uncertain in making the purchasing decision for the screen: should I get a touchscreen that connects via HDMI or the GPIO pins? Any advantages to either?
The safest touchscreen would be one that is built for the pi and has in-tree kernel drivers so you do not get stuck having to use a manufacturer's OS image that is likely to fall (or already be) out of date and may cause you other kinds of subsequent grief.
Unfortunately, navigating that realm is problematic because people in the latter category will still (fairly) advertise their screens as "made for the Raspberry Pi". The best thing to do is search around, especially here, for user reports about the screen.
The safest kind of generic screen would use HDMI for the display and a usb connection for the touchscreen input, since these are standardized and should not require any further drivers. However, I'm not sure how many people actually make such things.
In general avoid anything that refers to having to download drivers from somewhere and in particular things that refer to downloading entire OS images. The $20-30 you might save this way will for most people not be worth the hours of headaches and problems it may easily lead to.
In my opinion, one of the biggest advantages of using HDMI or DVI out is compatibility. Screens sold for the RPi that use a GPIO connector usually ship with a special image, and you're stuck using that image if you want to continue using their screen. Since their drivers are bound to a specific version of the kernel, you are often at their mercy when it comes time to upgrade the RPi's software.
The official screen uses the RPi's DVI output, and then the GPIO just for touch input. Other similar screens are more likely to use the HDMI port + USB. The primary benefit to the official screen is that the connector type is pretty compact.
The last thing that comes to mind is that GPIO controlled screens tend to stay on the small side. The only largeish screen I've seen that connects via GPIO wasn't very responsive when moving objects around the screen.
The Raspberry Pi Foundation sells a 7" touchscreen that connects to the Pi's on board DSI port over a ribbon cable, and connects to the Pi's GPIO pins for power. I would go with this one, as it is officially supported by Raspbian, and it sounds like the drivers are already built in, with support for 10 finger multi-touch. Here is a link. https://www.raspberrypi.org/products/raspberry-pi-touch-display/ This screen does not have built in sound, but that is a small price to pay for the ease of setup and official support.
