Small business looking to downgrade server

I work for a small buisness that only has 15 employees but occupies a huge building.

Our IT support is 100% outsourced and none of the staff other than myself are particularly tech savvy.

I feel like we're being massively over charged for our server and that it is simultaneosly both underpowered AND surplus to requirements, so looking for advice on how we could possible reduce reliance on it and also move to something cheaper.

Current setup:

£12,000 per year for the leased server (on-site) 15 networked Win7 PCs spread over 3 floors

Server is handling:

• Active directory
• Accounting software (cloud version is available)
• Building/diary management software (cloud version is available)
• 3x networked printers across 3 floors
• 30ish phones (possibly...). Our separate telecoms provider logs in to a management portal by accessing a local IP on our network but don't know to what extent this uses server resources
• Wired & wireless internet access throughout the building for sublet office spaces (devices not part of our network) for maybe an additional 40 users
• Storage of 500gb of shared files regularly accessed E.g. our marketing dept often work on large photoshop files that are stored on the server instead of local machine. I once had an excel file that was connected to several other excel files for automatically importing data into a master file, all of which were stored on the server instead of locally. IT told me this was slowing down the network and to stop doing it...

Proposed changes:

• Move all software packages to cloud based versions
• Move file storage to cloud (is something like onedrive / sharepoint actually viable/user friendly when it comes to opening/working on files that aren't saved locally or would there be other alternatives if not? Or is it exactly like dropbox where files are saved on local machine but immediately synced when saved?

A cheaper server would then only handle:

• Active directory (would Azure AD be a viable cloud alternative also? Is this more difficult to manage for IT providers?)
• Printers
• Internet access
• Phones (again, possibly... telecoms are beyond me)

Just looking for ball park figues for how much an adequate server should cost for the above. Is £12,000/yr as high as I think it is?

Management seem averse to buying any hardware when they can rent it for a managable but hugely inflated monthly cost.