I don't have anything. Totally starting from scratch.I'm not a speccer but do you have anything already like keyboard mouse OS and monitor ? This will help decide what components you need.
Brother runs Ableton on an i5 750.
you need lots of fast ram, and a mooooosive SSD
There are different options but if it were me and you was going to connect a midi controller/synth then i would recommend a dedicated sound card with decent ASIO drivers, i use the creative sound blaster z oem basic no frills model, you would also want to use the analogue cable that comes with the sound card, this helps greatly at reducing latency, there is nothing worse than hitting/striking the midi/synth key and have the sound lag a second or two later, it is very frustrating.