Both Paypal and a gateway will be "instant", in the sense that the payment is processed immediately and the funds are transferred as soon as the payment process is complete. This takes seconds. It's not like the customer will have to wait minutes, hours or days for things to happen.
The difference between Paypal and using a traditional gateway plus IMA is that with Paypal, you can then withdraw the money into your bank account manually. It is not done for you. With an IMA (internet merchant account), the money will be automatically transferred (settled) into your bank account after a certain number of working days.
Paypal may choose to withhold some of your money or limit your withdrawals if you get disputes that you do not resolve. Stick to the Paypal rules, resolve disputes quickly without it getting to the stage of a Paypal dispute and you won't have a problem.
Paypal is usually more expensive than using a gateway and IMA, depending on your turnover.
You do NOT have to get a gateway and merchant account from the same place. In fact, it often pays to shop around. I suggest you try:
http://www.cardswitcher.co.uk
Where you'll be able to get quotes to suit your business. The fact that you haven't been trading for 12 months shouldn't be a problem, but it will depend on what you do.