Just filling oil and nitrogen are going to be a waste of $$$..
It leaked out for a reason, you may want to address that before wasting money.
If you have a factory service manual and some basic tools it's not impossible. But a little suspension knowledge would be helpful.
There is a lot of information about this topic on the net. Just need to search it out. I replaced the seal head in my shock a few weeks ago. It wasn't hard. But, there were a few things that I didn't know prior to starting that I learned by doing the proper google searches.
Good luck.