🚀 go-pugleaf

I got an old IBM XT from work.  It has an 10 MEG hard drive, 5 1/4" disk drive,
and a monochrome monitor.  When I turn the machine on, it goes though the RAM
and usually stops at 480K the first time and prompts me to press F1 to continue.
Then the DOS 4.1 menu comes up.  If I go to the dos prompt and do a dir, it 
freezes.  So, I turn it off and back on again.  This time, it goes to 520K on   
the RAM and stops with 'PARITY ERROR 2'  and some numbers usually ending with
40000 or 70000.  Some numbers = two three digit numbers and the 5 digit number.
If I turn the machine off and on again, then the RAM check goes to 640K and
everything is fine.
Question:  Do I need some new RAM chips?  Which ones?  Is it possible that the 
chips need to be cleaned?  Any suggestions on where I can get a bigger hard 
drive very cheap?  and finally How much would I have to spend to upgrade this 
machine to CGA, 286 chip and 40 MEGs Hard Drive?
Keep in mind that this machine cost me $0  and I would like to keep my investment
in it under $200.