ok, Katie -
First, breathe - you guys will be ok. You CAN absolutely trust these people here. We have all been thru the fear and felt overwhelmed at first - OK? There are MANY, MANY smart experienced, caring people to help you thru this. It DOES get easier.
Sounds like your vet is NOT worried about ketones or any complications from trying the food change first - that is VERY good. I found the food on the "old canned food list";
http://binkyspage.tripod.com/CanFoodOld.html
Purina DM - right? That has only 7 carbs - very good!
Here is the link to the food info - new canned food list -
http://binkyspage.tripod.com/CanFoodNew.html
and the main page so you can read more:
http://binkyspage.tripod.com/canfood.html
You want a low carb food under 9 carbs, the lower the better - if the vet food is very expensive, there are many good ones you can get at the grocery store. Take a look at the list, but the purina DM looks good - if you can afford it and want to stay with it. Unfortunately, the vet's make money off it, so they push it.
I think it's good that your vet wants to try the food first, it really must mean her level is not too high - that's good!
But if you don't get a glucose meter and hometest, ask him for a Fructosamine test next time. (it cost me an extra $100) and it revealed a much lower level of BG's than the stressed out one at the vet. It can be rasied A LOT by the stress - kind of like getting a blood pressure reading from someone that is stressed, rather than relaxed reading, you know what I mean?
Meantime, keep urine testing her. You may just see that color change.
If you can't afford a glucose meter the board can send you one - look in the...list, in the feline diabets forum - "request for a meter"
Just remember - you are not alone, there is support here for whatever you decide to do, and there is no question too dumb to ask - OK?
Now give that kitty a belly rub for me!