Choice of head units and speakers are really up to you, and there are so many out there, its really a case of what you want and what the budget is.
If you are not looking to go too mad, and do not want the complications of an amplifier and/sub - then look around for some component speakers (seperate tweeters and x-overs) as a guide, you will notice that many manufacturers have a few different ranges - an entry level range, for those who do not want to spend any money and yet still think they can set the world alight with the sound, ignore this range, and look at the next level up.
Whether is be Focal, Clarion, Infinity, Kenwood, they will all be fairly comparable in price and I would look to spend around £100 to £120 for a decent set of components. These should take a small amplifier at a later date should you want to go down that route, but will at least sound awesome if powered only from an aftermarket head unit. Essential to keep the speakers from the same Manufacturer, by all means put the mid range in the front and the budgets in the back, if thats all you can afford, but dont mix makes.
The headunit is a minefield - loads of gadgets / mp3 etc etc, but again pick a good brand. Blaupunkt/Becker have truly excellent tuners but a little feeble when it comes to cranking up the volume, Pioneer / Kenwood have some good sounds and an excellent range of accessories to expand into, Sony appear have lost the plot over the last couple of years, but my favourite remians the Clarion units. They also do a 24 bit CD player (exceptional sound quality) and also have some models that have a 728 colour choice for the display, so that IT CAN match the miggy nightime display and also have a full range of bluetooth / nav add ons. Check out the Clarion website and give me a yell before you order anything !! Clarion will cost a few quid more than the others, but will be a stereo that will probably outlive your next few cars as you will definately transfer it from one to another. Other thing to point out that we thrive on Kenwood / Pioneer / Sony repairs here, but have not seen a faulty Clarion within warranty for nearly 8 years now !!
Double Din in a single Din hole? Don`t even go there on the Omega !!