There a few threads about upgrading wireless systems.
Lectrosonics have been field proven for years. You said that your older Lectrosonics have lasted 15 years. You don't say which models you have, but I will take a shot and say the older 187 series.
If you are using the 200 series, just send them in for tune ups and updates. There are still very usable. Just replace the ones that have been abused.
You do have a few options. If you are having problems or not, you should them to the mothership for tune-ups and updates. If you are not having any major inferences, this may be the best way to keep cost down. Just replace a few of the worst offenders. I still use my older 185 with no problems, and have seen the local stations still uses the older Lectrosonics.
I do like Zac ideas of reusing the transmitters and receivers, and purchasing new ones for replacements, but the transmitter takes most of the abuse in the field. I would send the transmitters in to be tuned up and updated. Also, they have to be 200 series or newer 100 series to work with the newer receivers. If you are still using the older 187's, these won't work with the newer Lectrosonics.
With only $1500 bucks per set to spend, it's not enough for Lectrosonic replacements. If you wanted to go with Lectrosonics, you would have to raise your spending limit.
The G3 is good for your money, but there are durability concerns. How good does the staff take care of their wireless now? The antennas are fixed, and not really field repairable. Most of the time, you have to send them back to the mothership. The cube has an XLR problem. It eats up the mic's XLR's. There is a thread about this problem, and don't know if the problem has been fixed. I have heard users breaking the LCD. Also, don't know the long term durability of the 3.5 mic connector. I did see a station go from G2's, back to 187's, but I never asked why. They use small cameras in the field.
For cost, the G3's are good for what you get. Most say upgrade the mic to something better. You can buy or have cables made to accept a mic or line level signal. The size is great if using small cameras. A neat trick with the G3's that you can use them as an IFB system. Never used one, but I guess you can control volume with the LED screen. Most users are happy with them, but wish they had Lectrosonics.
Overall, if your budget is only $1500 bucks per set, and you have to stay within it, and replace all wireless systems, go with the G3's. In the long run, they may cost you more for repairs, and time out of service. You would be able to have a few backups when problems happen. If you can mix, replace the bad ones with either G3's or Lectrosonics, depending on how much total money you can spend. I would send the ones that you are keeping to Lectrosonics for tune ups and updates.
Hope that helps