So, I have purchased a new VPS (virtual) server from a company recommended to me by a friend. Its pretty cheap and will allow me to set up comprehensive offsite backups to Amazon S3 with a reasonable total monthly outlay. I prefer this to a backup solution with the hosting company (the current arrangement) as its safer - if the hosting company goes bust then all the data is safely stored with Amazon.
I won't know exact figures yet until I finish the solution design, but I expect the donation fund to last more than a year, perhaps closer to 2 years.
I will be setting up the server over the next week and testing it out with a preliminary copy of the forum software. I am intending to do a migration upgrade to resolve a few minor backend issues which are preventing me from using the best antispammer module currently. I will take the opportunity to test out some possible changes to the forum and will invite a few people to participate in testing.
Once I am happy that the new server is working correctly, I will cut over to the new server (with a few hours downtime to copy the data over), and my friend can then decommission the old server.
The new server is lower specced than the previous server, but 55 euros a month is too much to expect users to contribute to, so we have to downsize and it should still be plenty fast enough for our forum purposes.