Great bargain there!
Theres plenty of threads already on PIDing.
You will get the nay sayers (usually probably never tried it) that will tell you that you should learn to use the machine in its current state and a PID really does not make much difference.
I will tell you that it makes a massive difference. After using a stock standard Gaggia for about a year then going to PID the difference is chalk and cheese.
The big benefits are consistency, much quicker recovery time, which leads to better steaming. No need to guess where the machine is at in its temp cycle, no flushing or chasing optimal temp.
I can taste the difference changing the temp setting on PID by one or two degrees all other things being equal.
Intra shot stability is only marginally improved, this is mostly affected by the groups ability to hold temp. However as soon as you start the shot the PID begins to try and rectify the error which does not happen without PID unless you like flicking your steam switch off and on.
I only loose a couple degrees on a double ristretto style shot in a minute or 2 the temp is relatively stable again.
I say work on upgrading your grinder first, this is where , (myself included) most people will notice a big change in there espresso