No, buying SACD does not guarentee "hi res." In an embarrassing PR gaffe, the lable had to admit that Nora Jones' "Come Away With Me" was made with 16 bits/44.1 resolution. As a matter of fact, it's an extreme rarity to see an SACD made from a DSD recording and mixed with no DSD-to-PCM conversion. Not to mention the fact that a huge amount are made from older analog mastertapes of sometimes dubious quality.
Nor will all SACDs play on your car stereo. If it's not a "hybrid" disc you can neither rip it nor play it back on a CD player. Unfortunately, you can never "safely assume" anything with the new formats. You have to read the label carefully or consult a competent review.
All DVD-A discs will play back on a regular DVD player. However, you can't access the MLP tracks except with a DVD-A capable deck. Most DVD-A's have a "high resolution stereo track," but not all. Most have a legend on the package describing the content, but again, not all, and there's no consistent labelling standard.
That said, your persistence and research will pay off with some amazing sounding discs. If you like the music, I'd recommend Steely Dan's Two Against Nature and Everything Must Go. I've never heard better sounding recordings in any format.
Some other great sounding ones (that happen to be good musically):
DVD-A
Beck Sea Change
Donald Fagan The Nightfly
Flaming Lips Yoshimi Battles the Pink Robots
Diana Krall The Look of Love and/or The Girl in the Other Room
Foreigner 4 (older but excellent sounding)
Crowded House S/T An overlooked pop gem, immaculately mixed.
Mark Knopfler Sailing to Philedelphia
SACD
Nickle Creek This Side
Alison Krauss + Union Station Live
Joe Satriani Strange Beautiful Music and Engines of Creation (although the latter is only stereo)
Pink Floyd Dark Side of the Moon
There's lots of others, but these are all good sounding ones that you'll probably enjoy listening to.