The shortest and nicest answer I can give you is this:

1. There are already mods that allow you to fly the non-buyable ships. Go get them, take them apart, and recombine them into a new mod.

2. Nobody's going to walk you through the entire process. Building a mod like that is a fairly large undertaking, and it requires that you already understand the fundamentals of FL modding.

But I have some good news for you. If you go get my Hfuel Tanker mod (which is on the Front Page right now), it includes some very simple and easy-to-understand methods for putting new ships into a mod, including having them show up in random encounters and Missions.

You could use that code to put all of the ships you're wanting into a mod, very easily- simply copy-and-paste the existing entries for those ships into the XML templates I've created... change their steering methods (for reasons I'm not going to get into, things like the Battleships used an older method of steering, which you'll have to alter, if you want players to be able to fly them)... re-label everything (so that instead of li_cruiser , for example, your script refers to li_cruiser_player_flyable or whatever)... create the appropriate entries in the XML to add the ship into a Faction or whatever... and there you go!

I make it sound simple- but it's not going to be easy, trust me. To make use of the XML, it'd really help if you understood what it does... and for that, you should really read the appropriate Tutorials about how to add a ship to a Faction, etc., so that you actually understand what you're doing here.

I know what XML does, I made an attempt at a mod awhile ago. It didn't go too well.

Everyone has one...
Some are one.....

Making a mod is a long and labourous process. At times you will literally want to throw your monitor out of the nearest window, other times, you will want to do a lap of honour around your house with your shirt over your head.
The best advice that can be given to anybody starting from scratch is this:

Read.
Read everything.
Then Read it again.
Then bookmark it and read it again.
The store for future reference and read when you are stuck again.

Also:

Never take anyone on their word claiming that something cannot be done. Everybody has a different brain process and look at things in a different light. All of the people on the forefront of modding now, once probably asked a question, were told it cannot be done, and then did it anyway.
Always try it.

Heres some starting links for you to bookmark: Compliation of Tutorials and New Modders Thread. 99.9% of the people here are very helpful and will either give you an answer or direct you to somebody who could. But you have to ask specific questions that aren't covered anywhere in those threads. The people who mostly get ignored are the people who say, I have an encounter and its broke, can someone fix it for me please?

Hope that helps