Any pluses or minuses to the older McIntosh mc275 compared to the newer mc275's IV, V, VI?

The MC275 is a great match for La Scalas or Klipschorns!  Series IV, V and VI are all identical internally.  IV and earlier have smaller spade-only speaker terminals while the V and VI have larger bullet/spade combo terminals.  IV, V and VI all have balanced and unbalanced inputs, but only the IV has adjustable unbalanced input gain.

Good luck & happy listening!

Adjustable gain on the RCA inputs may or may not be a big deal for you. They're primarily intended to permit control of a system with fixed line output (e.g. public address), adjust balance or compensate for a higher-than-normal output voltage from the preamp.  The XLR inputs are a nice feature; if your preamp offers them, that configuration can better reject noise and can be useful in certain environments (high EMF or RFI).

Series IV cannot be upgraded to the V/IV style speaker terminals; it's a totally different chassis stamping.  You wouldn't want to do that anyway because the MC275 is one of the most sought-after and collectible amplifiers ever made. When kept in pristine condition, used units can command nearly as much as new units.  In some cases (early models, special editions, industrial versions), they can go for quite a bit more than list price.  Tube rolling is where you can experiment, but be forewarned:  That can be both addictive and expensive!

La Scalas are indeed very efficient speakers, so you won't need to be cranking things up too much with an MC275.  That said, you will find the extra power provides greater control and more depth regardless of the music you're playing.  Make sure you spend some time fooling around with speaker location; La Scalas move a lot of air and have big cabinets, so distance from walls and resonant surfaces will really change how things sound.

Have fun!

MC240 would be another good choice for La Scalas.  At 105 dB @ 1 w / 1 m, most <100 watt tube amps work well with them.  I'd suggest you avoid SS amps though; they can often sound harsh with such an efficient speaker.

Very loosely speaking, more power = more definition and depth even when you use only a fraction of it.  You should research some of the threads here on the subject.  There are a number of good ones, and many posters are far more knowledgeable about the whys and wherefores than I am.

Do remember that the single most important thing you can do is satisfy yourself at a budget you're comfortable with.  Then you're free to concentrate on the music.  Which is really what it's all about, isn't it?