You'll be looking for a set of 165mm or 6 1/2" speakers unless you have an estate as the rears are only 5 1/4".
I replaced all mine on my old Lag with JBL's - not that tinny sounding crap you can get - they had a good "warm" sound to them & they had built in directional tweeters. The only akward part was having to cut out a small bit of the plastic cup the fronts sit in and the rears are a bit akward to get to. I was a bit miffed when I discovered they wouldn't fit my new Scenic
but I'm now using a set of 5 1/4" Infinity speakers and I think they sound ok.
Any tweeters will do for the dash as most will fit into the existing space - there's plenty of room. You might even be lucky enough to have wiring already there, but if not it's just a bit twiddly running the wires up the back of the dash.
Any head unit will do but it's mostly down to taste as with the speakers but you'll just need to pop down to your local audio store and have a listen. You can get some that you can connect to the controls at the steering wheel, although the adapter sometimes costs about £50 !
If you're looking for some decent bass a sub will be needed.
I bought all my speakers for the Lag on
- I got all 4 for £40 - BARGAIN I thought
Hope this helps - if you need any more info just shout