Here's what I've told my clients consistently over many years:
1. It never makes sense to purchase equipment that you don't need for tax purposes. Why? Because, the tax benefit will usually return somewhere around 33% + - of the cash outlay for the item, no matter how you treat the depreciation, expense election & interest expense. So, for every dollar you spend, you will receive 30 to 35 cents back in tax benefits. No a particularly good deal if your personal pleasure isn't worth 70 cents for every dollar you expend.
2. If the vehicle (or equipment) is operating & generating revenue & not costing an excessive amount of repairs & maintenance, then operating it for even one more year will significantly increase your bot***** *****ne by elimination the out of pocket expenditures for interest & principal on any loan on the replacement vehicle.
3. Since you are in the limo business, you are the best judge as to what the maximum mileage makes sense in terms of what you can achieve on a well-maintained vehicle. Every mile that you can drive a vehicle that you own beyond the payoff of any related loan, is a substantial profit that you wouldn't other wise achieve with a new vehicle.
4. You aren't buying a personal use vehicle, so you should be able to remove emotion from your decision.
5. One Company I had as a client, kept one "Pristine" vehicle, where they took extra particular care of in terms of the maintenance & repair, cleanliness, washing & waxing, etc. & kept it for their best repetitive customers and always employed that vehicle when there was a special reason to do so, beyond the mere assignment of the transportation engagement. You might want to consider a similar approach for one of your fleet of vehicles.