Hi Grandma G,
Some of the reasons people go towards more expensive treadmills are:
1) they are quieter
2) they have a better motor which will last longer
3) the treadmill belt is more sturdy which will last longer
4) they want to run or walk at fast speeds
5) they want special features like programs or adjustable inclines
6) better treadmills generally have a wider track so you can swing/pump your arms while walking for a more intense cardio workout
7) larger people (over 250 pounds) may be using the unit - some units tell you there is a weight limit of 250, especially if you are planning to run on it.
From what you say in your post, most of this doesn't really apply to you. Sounds like you have more of an issue keeping motivated to use it. (Although I must admit, if you spend a fortune, you do feel guilty if you don't use it which is somewhat motivating

) I would suggest that you stick with what you have right now if you are comfortable walking on it. Let it become a regular habit for you before you go investing more money. As you said, your health should be the prime motivators for yourself and your husband. There's many types of exercise but most people can deal with the walking on a regular basis. Even if you aren't racing along, it's better to walk slow and consistently than not at all.
Get yourself a clear glass jar and put it by the treadmill. Each time you use it, put a dollar in it. As you see the dollars accumulate, it might be motivating (might also make you a little guilty if it stays empty for too long). This way if you keep up with things, little by little you're building up a nice down payment for a new one by the time this one breaks down or you decide to trade up. Just make sure you don't have your treadmill on a carpet. The fibers from the rug will get sucked into the motor and will wear it out way sooner than it should.
The biggest problem most people find with walking is that it's boring. My husband likes to watch TV while he's walking. Me, I need music to keep me going. Other people like to read magazines. You just need to find a trick that will help keep you going and you'll be surprised how the time will pass. Other people like to keep progress charts - mark down how far you've walked each time and tally it up daily or monthly. It's makes you feel pretty impressed with yourself to see just how far you've walked in a month. Good luck to you and happy trotting!