Everyone is different so I’m not sure what things you will prefer personally but can give you things to consider…
Will you carry it on your back/body or will you hang it off the pram?
Will you also use it as your handbag or will you carry one of those separately?
Will you be the only one using it or will you send dad/grandma out with it on days they have baby? (Need to consider this if you’re going to be using it as a handbag and therefore it’s full of your stuff and then also how feminine you might want it to be for dad!)
Consider how easy it might be to wash. You might have to put it down in quite questionable toilets when changing out in public, or might spill some milk etc on it, so handy if it’s machine washable
How heavy is the material? Leather trendy ones can be heavy before you have even put stuff in it but might not be an issue if you’re hooking onto pram. If going to be carrying it consider how heavy and how comfy, look for slightly thicker straps to balance out any weight.
How easily can you get to things down in the bottom. You don’t want to be rummaging and rummaging for a muslin cloth one handed when baby has been sick or frantically digging to bottom for your wipes etc. It just needs to be easy…
Finally - budget - there are some really expensive ones and personally I just think it’s a complete waste. But that’s very much personal preference. All I would say is definitely don’t feel yoU need to spend a lot, or that £££= better/easier life, as it may well be the opposite (heavy awkward bags that can’t be washed easily)
Travel change mats are easy to come by so wouldn’t limit yourself to looking for one specifically that comes with one. Nice to have but definitely not essential. Definitely need one but can just buy one yourself