The thing about chihuahuas...
Chihuahua's are beautiful dogs but you must read up on the dogs character before you get one so that you can train it well. Chihuahua's are very temperamental and are constantly trying to dominate you... I have a chihuahua and a very well bred one... she's not aggresive at all but i've met plenty of chihuahuas that are... this is mainly because most owners do not know how to handle these dogs.
If you do get a chihuahua you'll have to be very firm with your training and must socialize the dog a lot (with people and dogs alike) or you'll be surprised that your chihuahua will end up bullying your medium sized dog and possibly nipping your friends in the heels. (I like to call chihuahuas chilli padi's!) Small but are little fighters...
Chihuahua's are very wary of strangers and can be very protective. They are usually fierce little pets when it comes to strangers, can and may end up barking at every little sound it hears... But they should never be aggressive.
A chihuahua is very good to have in a family but you have to establish a pack leader if you are to have one as very simply he/she will end up being the boss. Chihuahuas are not advised if you have young children in the house as they respond badly to teasing... (if the kids are calm and gentle then they have no problem)
As for potty training i believe you can do it in 3 months but the people around you have to reinforce your training or it will all just go to waste...
(Crate training is usually the easiest way to train your dog,
http://www.perfectpaws.com/crt.html)
Chihuahuas are playful and lovable dogs... they always want your attention and sometimes they're just happy being alone and chewing on their bones/rawhides.
There are many dogs for u to choose from... your idea to get a companion for your dad is great but like you said u want to make sure you get the right dog... :) Go to
www.akc.org/breeds and you can find out their rough temperament... if you'd like one in detail i can also write out a list for you so u know what you're getting into... :)
Hope this helps...