a Intel Core 2 Duo Processor T7400 2.16 ghz would fit too, but cost not much less
but a new processor is not really economical with prices of around $250 , but maybe its just because i live in europe where these early minis still sell for quiet a lot , so upgrading the processor and ram brings you above the price of a used mini 08 and 09 models ,and even close to a used 2010 model but i live in europe
so i would look into getting a working and "tested" used one , its more cost effective and processors are the most reliable parts in any computer really so can be bought used without much risk if the seller is serious and has testing facilities