If its just for the above mentioned then a second hand pc for about $500-$600 will do the job nicely (any p3 with a decent amount of ram (at least 128 MB, but the more the merrier (I'd reccomend 256))
As for the internet plan, if you don't want to go completly nuts, try some cheap adsl company (WHICH DOES NOT CHARGE FOR GOING OVER) with speeds at least 2x faster than a normal modem (lol). If it's just for forum browsing/email/internet 500 mb per month allowance is more than enough.
Second hand computers suck when you can get new ones cheapp here:
(not advertising, I just tell everyone who wants a new comp to visit these guys ... best value for money anywhere in sydney)
Also another point, make sure you get a monitor that is FLAT, they're much cheaper than they used to be ( < $300 for a 17") and if your going to be looking at it for extended periods of time its worth every cent you spend on it.