Hi, I was in a similar position to you, in that my 2001 Z28 arrived from Japan with a Monsoon tape head unit and both front speakers were rattling (blown).
I chose to replace my head unit with the CD version of the Monsoon, which was a factory option. In my opinion they sound pretty good and it was a straightforward swop out. I got mine from the classifieds section on the ls1tech forum, though there are always a few going on ebay.com...for not an awful lot of money...as long as the sellers are willing to ship to the UK.
NB: If you have a stock CD changer in the trunk, or plan on picking one up, then make sure you get a Monsoon that was manufactured 2000 onwards if you do buy one. Only those manufactured mid 1999 onwards (so safest to buy 2000 onwards) can control the CD changer.
An alternative is to look on this website:
http://www.crutchfield.com/
You can search via make/model to find recommended replacement head units.
Regarding the front door speakers, it seems (from what I've read) that they blow fairly easily and there are loads of threads on the US forums about what to replace them with. I researched this for a while and ended up buying a pair of Infinity Kappa 62.7i (available quite widely over here). Haven't fitted them yet, but apparently they are a good match both electrically (being 2 ohm) and physically, requiring no modifications to get them to fit.
If you are interested in replacing your front speakers with the Kappa's you may find this thread useful...
http://www.ls1.com/forums/showthread.php?t=29204
Good luck with whatever route you choose!