Whether you can remain on your page and so paypal transaction depends on the paypal business account you create. The account you created if it is normal, you can't ask paypal to redirect, you have to do it programmatically, when paypal returns to the payment page.
Second and third is also programmatically done. Are you a programmer or you got the project from someone. If your project code isn't shitty, then I am willing to do it. I lately had been given super-bad code, whose owners don't want to own their worse code and blame the next developer for their code's slowness. Yeah by shitty, I mean speed too. We are working over the internet. I have India's fastest internet, usually issue is on the other side, so.