Quakers (or Monk Parakeets) eat a similar diet to other birds. Seeds alone are not enough, but some seeds are good. They should also have some pellets, plus fruits and vegetables daily. Of course, fresh water is also required every day.
Quakers are very active and outgoing birds. They can be very noisy birds and are not considered good apartment pets. They are quite well known for being escape artists, so a secondary locking system on the cage may be required to keep your Quaker safely in the cage when you are not home to supervise. This fact is most likely why so many birds have escaped.
The flight of the Quaker is very fast, so a loose bird would be extremely difficult to capture. Even very tame birds will soon revert to wild when out in the open. Do not give up hope if your bird escapes though, as many are caught and returned home. (Please see the link below on Lost Pet birds to find the steps in recovering a pet bird.)
Because Quakers are such active birds, toys are very important to keep them occupied - both inside and outside the cage. Keep a variety of toys available to your bird and rotate the toys each week to keep him interested. Of course, check the toys daily to make sure they are safe, since parts or threads can come loose with active play.
Quakers can be good talkers. Their voices are not quite as easy to understand as an African Grey Parrot or Amazon Parrot, they are easier to understand that an Budgie.
If you can find a handfed baby Quaker, you will have a wonderful pet and this is what you should be looking for in a companion. A handfed baby will certainly cost more than a parent fed bird but will not take long before wanting to cuddle and play with his new human friend.