I was working on a trading bot before the Mt. Gox disaster.  My biggest problem was error handling and recovery... timeouts, partial fills, etc.  If you do a toolkit, that would be the most valuable part.

The Mt. Gox API, the trading algorithm... relatively easy compared to error handling.

(Since Mt. Gox came back online, prices have been relatively flat and my algorithm doesn't make many trades, so I haven't resumed work on the bot.)