Did you mean your son is 25 inches (about the 30th percentile for his age)? If so he will definitely outgrow the Snugride before he is one year old. He is too big for it when his head is within one inch of the top of the shell.
Integrated van seats are very safe for forward-facing. And you don't have to worry about how to install it! But your son needs to be in a rear-facing seat until he is at least one. It is safer for him to be rear-facing until he is 30-35 pounds, the maximum weight for rear-facing on most convertibles. He likely won't reach that until well after age two. So you will need a convertible seat.
One thing to consider also is how long you will keep this van. I think the prime age for using this seat would be about 2-6. But will you still have this van 5 years from now? If not, you may as well get a car seat you can use for 6 years. If you honestly feel you will keep the van and use its seat, you might want to consider a Britax Roundabout; it's a little cheaper than the Decathalon, but only goes until about 40 inches or 40 pounds.
Also, will your son ever ride in other vehicles? If so, you will need a seat anyway. There are cheaper convertibles such as the Cosco Touriva and Scenera that only cost about $40-$50, but they do not have much padding, probably less than your Snugride. A 5-point harness is safer than an overhead shield, so be sure to stick with the 5-points.
You might want to try leaving your Snugride in the car. Just put your son in and take him out like you would with a bigger seat. That could give your back some relief for now, although it won't make your baby any happier.
On the comfort thing, try comparing an uncomfortable car seat to something your husband is familiar with, such as bleachers, hard plastic seats, dining room chairs, etc. Would he want to sleep in one? Because that's what babies and toddlers do in car seats.
To sum up, I think the built-in car seat is very safe once your little one is about 2 years old. The built-in car seat is not very comfortable, and you must use another car seat anyway if your son ever rides in other cars. As far as lasting, you didn't list the maximum weight from the built-in, but I would think you would need to worry more about the van lasting, anyway.
HTH,
Julie