If you are uneasy about the money, it may not be a good idea. If the renovation cost and the offer together seem too much for you to handle right now, then it probably is not a good deal to invest in the building.
There comes a lot of work maintaining the place as well, I am not sure how big the building is and how expensive property taxes are there, but the site alone is enough to handle right now, and I don't see a big need for having a team, do you?
In the future maybe, but this may not be the right time. Now, since you have already made one offer, than you obviously are interested and must have very well weighed out the pros and cons, because of this initial offer you made this makes me feel like you do need the space and that you do want to move the NG office out of the basement and actually get a team going. In that case, it may be an investment worth going for. Having people work for you, while you work on on the business is always more beneficial if not - if you can keep up with taxes and other newly related costs.
Really think out the money factor here. I am not your accountant, so I don't know what you make exactly, though I have idea, and I think you are in a position to go either way. You are comfortable right now, maybe better, but if you expand slightly, you could very well be just as comfortable in the new office and could in fact turn in better earnings as well as efficiency both for yourselves and us.
Sometimes the risk is worth taking, but sometimes it is not. I like 3:1 odds, but 10:1 odds is not my game of choice. So, do what is best for your financial situation, but don't rush. Most likely, you won't be even able to outbid the neighboring building - I think.