You don't have to do that, you can and if you would it will be absolutely bulletproof for furture (no more problems and dealings with crappy gaways), but you don't have to.
Just use API of current Gateways:
https://en.bitcoin.it/wiki/How_to_accept_Bitcoin,_for_small_businesses#Merchant_Services
there is eight (8) of them and they all share the same medium - Bitcoin. The beauty of it is that once the customer gets used to the bitcoin you don't have the problems you had with PayPal - educating customer to move on from this piece of sh*t to another payment processing gateway and then to another and indefinately.
Anyway just to make it a bit sweeter for developers here is my offer for the plugin:
20 USD for the plugin using API of current gateways (see above, at best choose one that is not US based). I assume anyone who knows how to code can do it in a few minutes/hours.
100 USD for plugin solution that works directly as standalone with this script
It should function like this:
1) pre-generate several wallets for the script to use (let's say 10 wallets, with option to add more later)
2) once customer pays, to protect your and his privacy random the wallets and offer him one (another customer gets shown differnet wallet)
3) calculate how much bitcoins customer has to send from his wallet (BTC exchange rate fluctuates quite heavily) to get his account upgraded. Ie. Your 1 month premium account costs 10 USD and with current rate of 27.4 USD per 1 BTC (http://bitcoincharts.com/markets/mtgoxUSD.html) you will ask him to deposit 0.364963503649635 BTC (10/27,4)
4) Once the transation is confirmed (by peers - see wiki how Bitcoin works) his account is automatically upgraded (like it was if he used PayPal)
--- For customer it ends here - he gets account upgrade, I got the bitcoins
5) Bitcoins now sit in site administration's wallets ready to be moved - to be exchanged for fiat money (at exchanges) or just spent directly - there is plenty of options: https://en.bitcoin.it/wiki/Trade
http://www.bitcoindir.com/
Btw. as Mega's resellers PayPal accounts has been frozen they started accepting Bitcoin: https://bitcointalk.org/index.php?topic=139566.0
Once a team writing a particular filehosting scripts (yes, there is quite a competition out there) will add support for Bitcoin this particular script will immidiately become really big thing - all the problems with payment gateways will be gone forever.
Here is some place to start: https://en.bitcoin.it/wiki/PHP_developer_intro