How to choose my first PLC?

I'm about to purchase my first PLC ever.
The use of the PLC will be to connect between some production machine to a manipulator (robot), and also some outside accesories (i.e a piston, an ionizer, vacuum etc.).
The PLC should get input from the machine (i.e mould open, mould closed etc.) and send the appropriate pulse back to the manipulator, but in the same time to get inputs from the manipulator and activate the accesories.

As for now, I don't deal with analog inputs, only 1 and 0s (there is current or there isn't).
Except of the amount of inputs that I will need (from both the manipulator and the production machine) and the amount of outputs I need, to send signals to the manipulator and for activating the other accessories, what else should I consider?
To my understanding, I need NPN inputs (to receive signals from both the machine and the manipulator) and PNP outputs (for sending signals to the manioulator and activating accessories).Is that correct?
What else should I consider? What is better for my needs? Transistor outputs or relays? Maybe transistors and connect them with external relays if needed?
Do I need to worry about the speed of the device (I don't think so)?
What about counter and clocks? I don't see the need of and internal clock rightnow, but is it worth spending more for maybe future applications?

Sorry for blowing you up with questions, I'm a total newbie and have no experience with those devices.
Hope I was clear enough to let you help me.

Thanks,
Yair.

A good place to start is http://plccompare.com.

What is best depends on IO requirements, programming language preference, distribution of your IO points, type of IO, physical size restrictions and your budget.

Velociraptor, thanks for the link. I'm currently trying to figure out exactly what I need, that's why I'm consulting you.

Stephen - Can you please explain what is PWM?

And another question - as long as I don't deal with sensors yet, Can "convert" NPN to PNP just by using the right symbol (i.e ] - [ or ] /[)? They're just some electric pulses coming out..?

Do I really need PNP or what I need is NPN? I'm kind of lost with that matter...

EDIT:
OK, I think I git the idea. NPN output will only close the circuit but won't provide any power, I need to connect the output accessory to a power source?

Thanks!

The type of inputs and outputs you will require will be dependent upon the devices you choose. A relay is a contact closure.

The symbols you are are talking about, refer to programming for [/] normally closed and [ ] normally open contacts in programming, ultimately determining current flow of the ladder diagram.

I would suggest using some of the information on this site to get familiar with the terminology...

http://www.plcs.net/contents.shtml

Pulse-Width Modulation (PWM) is a highly efficient and convenient way of controlling output voltage to devices with large time constants, such as controlling the speed of a DC motor, the power to a heating element, or the position of a proportional valve.