Service contract expired, what can you do about the oil leaking from mixer?

mine did the same thing (we moved it to the spare bedroom during the kitchen remodel, it ended up sitting about 7 months) - we used this YouTube video to replace the grease https://www.youtube.com/watch?v=8eKHVvNX5eY - the grease is also available here on Amazon https://smile.amazon.com/KitchenAid-Stand-Grease-Enough-Repair/dp/B00OSC174E/ref=sr_1_1?ie=UTF8&qid=1485645554&sr=8-1&keywords=kitchenaid+mixer+grease - it's not hard to do and keeps your investment in a Kitchen aid stand mixer going!

does it work well with pasta attachment?

My old Kitchenaid pasta attachment broken last Dec, now I take another inexpensive one from FavorKit brand, 3 functions set only cost me $70. No problem till now after 6 months use

what is the main difference between the artisan series and the 500 professional series? also which one is newer?

The wattage of the motor is the difference. The artisan series motor wattage is 325; 500 professional series 450 and the 600 professional series is 575.

Will this Kitchenaid attachment work on this model? http://amzn.to/2lz5ema

From what I've read about your model they recommend #9708471/ a 'C hook' / I wouldn't use anything that is longer than 8 1/2 inches since your bowl is listed as 8 2/3 deep