If someone had installed a custom/homebrew ELS, I could see that being a bit more worrisome. An ELS does depend on the mechanism for leadscrew to spindle timing. If it was a homebrew ELS rather than a well known endeavor (such as the Clough42 design) that would be a challenge to debug even for someone with coding experience, if problems came up. Especially if you didn't have the source code. I suspect some miscommunication about this created confusion between VFD and ELS.
A VFD conversion, I would not hesitate, unless it was a no-name knock-off. In which case I would price in a replacement known VFD in the purchase price.
I have a Monarch 10EE lathe that had the electronics replaced with a pretty poor DC controller. I started to work up an improved microcontroller-based DC system which worked well, but then realized the DC motor itself is in rough shape. So I'm sitting on the fence of getting the DC motor repaired or replaced, or doing a VFD conversion. In this case I think a VFD conversion, if done with schematics and to reasonably neat workmanship standards, would be a resale win, but may still pursue the DC option.