SBS2011 Performance Issues (High RAM Use & Disk Thrashing)

Have an SBS2011 in a very small environment I was tasked with Administrating (remotely with occasional onsite visits).

The server has 1 medium-high use user (6 days a week) and 4 occasional/very light users (only 2 ever connected).

Spec: Xeon E5506 (Quad core), 12GB RAM.

Think the Disks are 4x SATA in RAID5.

Running SBS2011 Standard. Kaspersky Small Office Security.

The Server was installed and configured for 3 users as an e-mail box by an external Support company (who still cover it). For over a year it was left running with no Administration (Updates were set to manual) before I was asked to take a look at it (soon after starting with the parent company). It has always had stability and speed issues.

One 3rd party installed an 'FTP retrieval' program which runs nightly to upload a stock report and download an EPOS document in .CSV format. Another 3rd party installed a Stock Database program (running on Postgres) on the Server for the client machines to connect to. These 2 things were done immediately before the speed/performance issues because a larger concern (and it became my task to resolve).

I know Exchange Databases (SQL) and IIS Databases are set to use any available memory (for caching) and then release it when it is required by other processes; this process did not seem to be functioning on this server, so I took several steps (below) to set (& restrict) these databases not in use.

Steps I've taken so far:

Install all of outstanding updates (70+ of them).
Changed the router and lock down the router (and Servers) Firewalls (previously very insecure).
Limited the Exchange Database Store - http://eightwone.com/2011/04/06/limiting-exchange-2010-sp1-database-cache/
Fixed / Reset Sharepoint (after updates) to get the backup working - http://social.technet.microsoft.com/Forums/en-GB/smallbusinessserver/thread/94c5f178-f020-4d0f-ba7c-11c415d0d862?prof=required
Looked into restricting Sharepoint (it's not used) as disabling it is a bad idea and restricted the SQL databases it uses - http://msmvps.com/blogs/bradley/archive/2005/02/04/allocated-memory-alert-revisited.aspx

Troubleshooted IIS Databases to find which I could restrict - http://serverfault.com/a/195566/106401
Set Recycling settings for the IIS Databases quite low for the ones not in use (Sharepoint, WSUS) - https://www.microsoft.com/technet/prodtechnol/WindowsServer2003/Library/IIS/0e570911-b88e-46be-96eb-a82f737dde5a.mspx
Disabled WSUS - https://msmvps.com/blogs/bradley/archive/2011/01/31/so-you-use-another-patching-tool-and-don-t-want-wsus-on-your-sbs-2011.aspx

And I'm STILL getting performance issues with this Server! Currently 'Datacollectorsvc.exe', 'EdgeTransport.exe' and 'mssearch.exe' are using more RAM than I would expect (upto 99% in use).

I installed and ran SBS 'Best Practices Analyzer' and it does not report anything - http://social.technet.microsoft.com/Forums/en-US/smallbusinessserver/thread/35db755e-6c0d-440a-8216-e21ce1e0a7b0/
Attempted to run the 'WSUS Console' Cleanup Wizard, but as I previously restricted/disabled parts of WSUS it doesn't run.

What should be my next steps?

I realise that fitting more RAM/Virtualising Exchange away may be the best solutions, but my Directors won't want to fund more money on Hardware/Configuration/Support as this Server has been a complete let down since installation (We run 4x Ubuntu servers with 10x the workload, 10x the users on lesser hardware experiencing no performance issues).

Note: This is the only Windows Server we have, my previous experience was with multiple NT4.0/Netware/Windows Server 2003 (and not any SBS installs).

You can find many of my questions/solutions/answers and troubleshooting on Super User - http://www.superuser.com and Server Fault -http://www.serverfault.com (both sites part of the Stack Exchange Network).

DNS Role Failures: http://superuser.com/q/410738/102661

IIS High Usage: http://superuser.com/q/414440/102661

Disabling Sharepoint: http://serverfault.com/q/415846/106401

Server 'Detailed Network Reports': http://serverfault.com/q/432122/106401

IIS Restricting/WSUS: http://serverfault.com/q/450038/106401