28561 router IOS upgrade

Question

Hi all,
Total noob, but learning.
I bought a second hand 2851 router (CISCO2851-HSEC/K9-REF)
software:
ROM: System Bootstrap, Version 12.4(13r)T, RELEASE SOFTWARE (fc1)
I'm getting a boatload of memory fragmentation errors, then it failed to ROM image on 'unknown error'.
From what I have read from searches, all point to upgrading software.
How do I get this?  How much does it cost to get to the unified version(s)?
Does the upgrade see what licenses/features I have installed and automatically enable them? 
Am I allowed to use the software on the Catalyst 2960 and 3750 I have too?
And is 512MB of DRAM the max this router will take?  At 256MB now.
Cruising the website is cumbersome and left me more confused then before I searched.
TIA,
Dave

Answer 1

Don't worry, you are in good hands, just make sure as a gesture of gratitude, please keep rating the posts which you believe have helped you :-)
"I'm getting a boatload of memory fragmentation errors, then it failed to ROM image on 'unknown error'."
This is not an encouraging news, could you please unseat the memory and put it right back and see if these err messages go away? These messages show that, some kind of either chipset issue or even worse, Motherboard issue :-(
IOS corruption may also contribute err but I have mostly seen either a crash of the Boot process or no booting at all, and the router gets stuck after the POST.
You can download IOS from the Cisco website as long as you are not using the router in production and/or for profit from a business, all you need is a valid CCO ID and a Smartnet (ESW) or UCSS contract.
That should be able to help you to download the software and use it for non production environment.
HTH