Before you hire anyone else, find out why the rest of your dev team resigned and fix that issue. That may well inform your decision about who to hire next. In particular, while it's unlikely that the whole team would resign due to your one remaining dev, it's just about possible - in which case you'd be finding a new dev to start with.
Hiring someone and then finding they quit soon afterwards is going to be expensive and time-consuming. You'll also find that good developers will be reticent to join a company which everyone else has just left, unless you can assure them that whatever the problem was has been fixed.
Once everything else is fixed, if you've still got the situation of "budget for one more, and still got one dev" then you should look at what your current codebase is like. Is it a disaster waiting to happen, which could really do with some good testing to shake out all the problems? If so, hire a tester. Is it basically okay, but you need to add new features on this solid foundation? Hire a developer, but explain to them that they'll be expected to do testing as well.