I'm not interested in selling right now. I am running and growing it over the next 6 months, at which point I will look for a buyer.

Marketing costs are around $350/month for some simple BuySellAds spots.

I think the ideal purchaser would be a web development company or enterprising web developer (PHP programmer with frontend skills too) who is willing to pay ~$100,000 up front for a growing and proven business, the client leads, etc. However, I'm not sure if this is possible to find.

I think a buyer will be findable. Not easy but surely one or more are out there.

You have enough business savvy to find customers, use the same savvy to find buyers.

whats the language/platform of the software? how many customers/users are there currently?

PHP. ~200 active clients.