Depends on what you want from the car. If you're one of those types that is considering buying one simply because you've heard they are cheap to buy and you want the "look" and "status" that a Euro car gives but you dont actually LIKE the cars themselves particularly then please save yourself the trouble and avoid them.
I'll have a stab at answering the questions anyway for what it's worth.
1. Market Price - The PassAt and Bora are two very different cars, and Skoda has an entire separate range so you cant give one market price for all. Sticking with the Passat and the Bora, Boras retail for between 1.6 and 1.8 Million realistically. 1998-2001 (pre Facelift or B5 Model) Passats realistically should be priced at between 1.2-1.6 Million although asking prices vary wildly. The 2001-2005 Facelift (or B5.5) versions go from anywhere between 2.0-2.5 Mill depending on condition, seller, buyer etc.
2. Spares Availability - Much better than before with several importers who have most of the major parts in stock. Body parts might be a bit of a hassle but definitely not unobtainable. They will be more expensive than Toyotas and Nissans.
3. Fuel Efficiency - Not really
4. Secondhand Market - Good cars are in high demand but difficult to find. Most of these cars have already depreciated to their lowest point so any massive drops in value are unlikely unless a major shift happens in the overall market like a huge tax drop. The actions of the agents and lack of parts have given them a bad reputation hence the low prices. But now that the parts problems are mostly sorted and since there now is an Independent Specialist for them, things are slowly looking up.
Essentially, if you're just looking for something big, cheap and flash that you can abuse and run on the cheap, go buy a "Jaguar Light" Hyundai Sonata and get it diesel converted.
Passats/Boras et al can only properly be run by people who understand that they will definitely have random issues and are prepared to deal with these because they appreciate the many other qualities that make these cars stand out from the Allion/Axio/121 crowd. If used that way, with the proper care and maintenance, they will serve you well. Otherwise they will become a nightmare that will make you curse everything and everyone ever connected with them. So consider yourself warned. When you buy one of these you aren't only buying a car, you are buying a hobby as well.