Hi Michaela, Welcome to the forum!
The Accu Chek Combo is both a pump and a BG meter. The meter is almost identical to the Accu Chek Expert of you've seen that one, it just has an extra choice on the menu to link it to the pump. It uses Bluetooth to link to the pump, so when you've done your BG test you can tell it you want to do a bolus, enter how many carbs you are about to eat, confirm that you want to do the bolus and it will calculate the total bolus for the carbs + or - an adjustment to bring your BG back to the middle of your target range and then tell the pump to give you the bolus. Obviously you have to set up all the ratios to enable it to do the calculations correctly, and you'll probably find that you need to tweak them sometimes but it makes mealtimes very easy! You can also do extended/multiwave boluses, temporary basal rates and even set up and change your main basal rates all with the BG meter, in fact you hardly ever have to touch the pump at all except when you are refilling it.
If you are thinking of a CGM rather than a normal meter though then they come separately and don't work with the Combo, you have to get either the Animas Vibe or the Medtronic Veo, which don't have the built in BG meter. It's also very hard/almost impossible to get NHS funding for CGMs (depending on where you live it's hard enough getting a pump!)
We have only ever had the Combo so can't comment on the others, we love it but you'll find that almost all pumpers absolutely love their pumps and would never give them back, they are life changing. Good luck, hope you can get one!