So I watched "Spearhead From Space". It's a little weak, but I can tell the series has possibilities. The Doctor didn't get nearly enough screen-time as I'd hoped, but it seemed like a good introduction to the franchise. Since he's got partial amnesia, I can learn things as he does, which helps a lot when you want to get into a series like this.
Don't count on the amnesia lasting; that's just a side-effect of the regeneration. He's pretty much status-quo by the next serial.
That said, the purpose of any given regeneration is to set up a new status quo and act as a friendly jumping-on point. #3's run, in particular, is a serious departure from the rest of the series; most of it's earthbound and (then-) present-day.
It DOES introduce a lot of aliens and races for the first time, including the Autons (as you note), the Silurians (an excellent serial BTW), and the Master (though his backstory is that he and the Doctor have known each other since school, so his first appearance doesn't feel much like a first appearance). The Brigadier is of course a returning character, as the story makes clear, but his reintroduction tells you everything you need to know about him.
The plot is serviceable. "Alien intelligence threatens to replace government with department-store mannequins" is just the kind of silly sci-fi boilerplate that it takes to get the ball rolling.
Which is exactly why Davies used the Autons to kick off the current series in '05.
The characters are fairly decent. I was afraid the doctor would be dour and serious as the stills of Jon Pertwee I've seen have led me to believe, but he comes across as easygoing and mischievous.
Most iterations of the Doctor freewheel between lighthearted eccentric and intimidating master of time and space. Pertwee is a favorite of mine; he's a complex Doctor, alternately cranky, arrogant, apologetic, curious, sanctimonious, driven.
And he's the Doctor most likely just to fucking Venusian Judo-Chop the bad guys. Funnily enough, the two manliest incarnations of the Doctor are the one who wears black leather and the one who wears red velvet.
The Brigadier is the consummate military man: a bit put-upon by the Doctor's eccentricities, but professional.
The Brigadier is probably my all-time favorite companion (and to hell with quibbling over whether or not he "counts" as a companion). He really is the best foil the Doctor ever had; they're so alike and so different, the Brigadier just as stubborn as the Doctor and the two of them often philosophically opposed. And Courtney is JUST. SO. GOOD.
I particularly liked Caroline John's portrayal of Liz Shaw. She's cynical and acerbic, which makes a nice contrast to the Doctor's cool intellect.
Again, I was surprised, on reading her obit the other week, to find that she was only in four serials (and that I've seen three of them). You get a good enough feel for her character in just a few episodes that they can pull a reversal in Inferno (Doctor Who's version of Mirror, Mirror) and highlight the similarities and differences in Parallel Liz.
I like Jo but she's just not the same, and in terms of Female Empowerment she's definitely a step backward after Liz.