It's a SaaS on heroku that hasn't really been updated in 5 years. It's not deployable (without dirty hacks). I know the CI is red. Runs on old versions of software that don't get security updates anymore and aren't supported by heroku, etc
From a purely tech perspective (not business/sales perspective), is there anything that is worth doing to get the business to sell more easily or for a higher price? Or does the tech side not matter at all and it's all sales/revenue/customers?
A) for the customer base. This us when the buyer is a competitor. Acquiring you is just a step to converting your customers to their product.
The value will depend largely on how many -customers- you have (people who give you money) versus -users- (people who -gave- you money once, or never).
B) for future profit. The seller is selling something that generates cash, the buyer offers some multiple of that cash up front. The buyer may have plans to increase the cash, the seller takes less cash now, but equally gets time back.
In this scenario improving the tech -may- result it a better price, (but probably wont). What it may do though is widen the buyer pool. Getting the product right is easier for you than them, and uplifting it may make it more desirable.
On the one hand painting the house makes it sell for more, on the other it matters little if the house is being demolished anyway.
In other words the answer to your question is that you need to know who you are selling to (what market) and what they are buying.
I helped work on a startup a few years ago that changed ownership. Like you're saying, documentation wasn't great. After hiring devs to work on it that couldn't make any progress, they hired me to make some emergency changes. I also offered to improve their documentation and setup a proper release process so that other developers could work on it.