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kernel drivers so you do not get stuck having to use a manufacturer's OS image that is likely to fall (or already be) out of date and may cause you other kinds of subsequent grief.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.