@vyral_143 - I guess
@Ethan_Hunt has answered most of your queries. I will add my thoughts on PSN and Keyboard + Mouse.
PSN as a service is free and you can play most of your games online for free. The 2 conditions that apply here are -
1. Some games include codes, not a problem if you are buying a new game but on a used copy you will have to buy the code unless it is an EA game as they have dropped the requirement of codes a while back
2. Sony can close servers i.e., kill the multiplayer support. It happened on the Resistance series and there was some talk about that happening to Gran Turismo 5 as well. I tried GT 5 2 weeks back and it was still up. Haven't tried any of the Resistance games in a while.
So if you have a decent connection around 1 mbps you should be fine. Please Note GT5 works well with 512 kbps but Resistance 3 used to hang and reset the PS3.
Keyboard + Mouse -
Keyboard and Mouse work on the PS3 but not in games. I have tried it, it doesn't work.
Aiming with the Analog sticks will take some time getting used to but hang in there for the first couple of hours.
There are 2 thing that help out here -
In most games you really don't have to be very precise and provide ammo like garbage.
Some games offer auto lock feature. Some do it very subtly providing some assist. I know as a gamer you want to go gung ho and do it yourself and you can by bumping up the difficulty.
PSN or PSN+?
PSN+ definitely. Apart from games given out monthly for the fee of a single new title for your console. It stupidly also gives you games for consoles that you don't own. I used to this for Vita and then when I bought the Vita I downloaded the games that I bought months before getting a Vita. I am doing the same thing for PS4 now and I don't have a PS4
PSN+ also gives you full games with a certain time limit. The time limit is of course short enough so that you can't do a speed run.
PSN+ also gives you some discount.
PSN+ also gives you 1 GB of online storage which is helpful as it allows you to backup game saves on it.
PS3 also allows you to...
You can download and transfer files from your PC so you have a backup of it and need not re-download every game.
This helps considering at times downloads on PS3 gets corrupted or certain games pause download if you are signed into your PSN account.
If you have a few friends who also own a PS3 then you can all download and share the games providedyo u have all bought them or got them free from PSN+
Thus you can back up and build a collection of virtual purchases.
NOTE - PSN+ online storage or the PS3 itself has an odd bug. When you select multiple files or all then some saved games are greyed out and don't get uploaded. I found this day before yesterday. I was syncing my saved games and saw that Vanquish was amiss. Then found out a few other games like Star Ocean and Driver SF also had similar problem. For such games copy 1 saved game at a time.