#1 - why not just check ebay and buy a use one. Much cheaper and possible also working and identical to the original defender.
#2 - Now sure about black light, but all you need is your typical cheap homedepot counter florescent light for the marquee.
#3 - that pc will work fine for defender and like games. Also consider an older version of mame, like .6x or .8x. You can run either OS and get full speed. Ideally, you best bet would be either DOS, win98 or linux.