Hello! First time poster here. I'll get my questions out of the way and introduce myself after.

My father has agreed to allow me to be an authorized user on one to all of his 3 credit cards. I need help deciding which one(s) to become an authorized user on. 2 of the cards are about 8 years old each and had quite a bit of responsible usage in the beginning but have been sitting idle for a couple of years with a zero balance but are still active accounts with over $5,000 limits each. His third card, which he opened just a couple of months ago, he's been using for small purchases up to $500/mo and paying it off on time, every time, before the due date. He has agreed to allow me to be authorized on any/all of the cards. Which should I choose? Will the 2 longer ages accounts with nearly no recent usage do anything for me? What if he or I began using them in small amounts? I'm assuming that the newest account with the highest limit won't help me much because of its short age, combined with the very short age of my personal accounts. (See below). I'm open to all suggestions!

A little background on me:

I currently have a score of 620, was 480 just 1 year ago. Make close to 100k/year. My credit limit is $700 between my only two credit cards. So only two open accounts, 6 months total account age. My first one was opened 6 months ago and has a perfect payment history with regular usage, keeping the utilization below 30%. I pay this card off every month. My second card is less than a month old but I have not used it yet. Plan on using for very small purchases and paying off every month. I currently have 2 old collection accounts open from about 5 years ago, totaling under $1000, that I'm going to be doing PFDs on. I'm planning on also opening a small personal loan with my bank in a couple of months, probably 2-3k, and paying off slightly higher than the minimum over the 1 year term until paid in full. My goal is to get my score to over 700 by the end of the year and get into a house. I'm hoping that becoming an authorized user on one or more of my dad's credit cards will get me into a good spot, but am open to suggestions.

Your time and help is greatly appreciated!