[cynicalbastard]

at 18 months, you are either a rocket ship, or should be financially and mentally equipped for the long haul of a 10+ year bootstrapped business.

but it sounds like you are neither, so you should take the offer, and maybe try to get some contingencies in there like a modest signing bonus and health coverage so you are not financially over-stressed on day 1 of starting a new job, which puts the work and yourself at risk.

if they are asking you to work on the same thing, try to get some performance-based bonuses also. it will motivate and rejuvenate you to continue your work (literally).

paying back your investors in full is probably wise, in case you forget all about this fun little experience and decide to do it again (you will).