I think if you look at it from a capacity planning point of view, you would need to move on to paid hosting in order to grow the site anyhow. Few reasons for me saying that;

The busier the forum gets the busier your SQL database will get. That'll give you another problem.

Members can upload as much data as they want through the forum software. If you had 500 members that chose to upload as little as 5MB each, there's 2.5GB of storage you need to find. Not to mention the associated bandwidth of other members downloading the data.

If you had say 4 members wanting to download each of the 500 members 5MB of data they've uploaded, there's 10GB of data transfer.

Donations can be a good thing, and I'd certainly be willing to donate. All I'd say is that there may be tax implications there. I guess you'd have to prove to the HMRC that every penny received gets spent on maintaining the website, otherwise no one would pay corporation tax. Suggest you read up on it or at least speak to your local tax office before making a decision on that.