I am not too sure of the prices, because I dont know the condition the comp is in .. but it seems OK! Its about at par for what you would get on ebay (I was just able to do a rough check), perhaps abt $80-100 more than what you get on ebay.. I'd say its worth it if you know the seller, and he will give you some form of a warranty than to deal with the many factors of some seller on ebay. You could perhaps get the seller to reduce a bit
As to the system itself it is great! Don't be fooled into doing a Mhz comparison between x86 (Intel/AMD) vs a 64bit UltraSPARC .. the x86 will be left in the dust.
Linux will work great! Go for Solaris should be fun.. So will TruUNIX

or AIX! Either way you will get performance that an x86 will find hard to compete with .. even at the 300Mhz range!