That is a damn good start! You have your list, you have a design and you have a plan!

"Am I completely mad to try and build a spinner with no previous robotics experience?"
- Yes, but that didn't stop me from building Conker 1 ;-)

"Is hardox a good choice for the bar and does the grade matter?"
- The higher the better, 450 minimum - 600 at Best. After tossing a basic bar design through Solidworks, you can get your 500mm bar in at 12mm thick, 60mm wide, with a 90mm dia hub in for around 2.6-2.7Kg.

"Could I get away from only having spares of the cheap things e.g bearings/drive motors? What parts are prone to breaking? Do brushless motors have mounting holes at the bottom? Is it a good idea to mount only to the bottom, or is supporting both sides a better idea?"
- In an ideal world you would swap out the stock bearings in the brushless, glue in the magnets, put in a Titanium shaft and then have a bearing to support the back of the motor as well. That said many machines run stock motors and just have lots of spares. The NTM's are so cheap that its more effort/cost to reinforce them than replace them.

"Is the stall current of a drill motor over 30amp? Is the ESC i’ve mentioned be able to handle the stall current or is the Tz85a a better choice?"
- TZ85A every time. If you look after them they will last you several years and several robot iterations. Just check the insides every so often for swarf and loose components

"Where do people normally buy hardox from? Struggling to find any online sellers that sell something other than digger wear plates."
- That is a tricky one. I got the Hardox for my first weapon from JL Steel in Scotland. They were very kind and provided me with a plate for my weapon (20mm Hardox 500) and my armour (4mm Hardox 450) and asked I donate £60 to MacMillan Nurses rather than paying them. However - http://3.bp.blogspot.com/-65GE6TQ2E_.../s1600/041.JPG - That is what I did to a 12mm thick bar spinner last time I met it in combat so you will want a spare.