It's no spoiler that you make money throughout the game. So that means you'll have a lot of money when you're done with the main story, and if you then start doing the assassinations, you can double, triple and more with it.
So stay away from the Lester missions with Franklin.
And here's a great stock tip for those missions:
Lester will have you kill a major head at one of the companies. The rival company will rise a lot due to this, so before you do the mission, invest all the money you have on all characters. That means when you start a mission, and Lester tells you which two companies you're dealing with, like "kill the guy of <this company>, that'll make the stock of <that company> rise". Wast yourself, invest all your money on <that company>, restart the mission. Do it, wait a few hours for <that company>'s stock to peak. Sell everything, and invest it all in <this company>. <this company> just saw a huge hit in their stock, but it'll rebound over the next days of in-game time. Let it go back up, and you can triple your money.
Yup. About the airfield they set you up to buy with Trevor, that's actually one of the only worthwhile investments in the game. If you do somewhere between 10 and 20 plane-missions, you'll get more than you put in. I forget what the number was, but if it cost 100k, and you get 7k per, means you'll have to do 15 to make money.