Hi David,
I routinely install landscape lighting as part of my business so here's some thoughts.....
LED is the way to go. Much less energy consumption as you said plus you can install in daisy chain without much loss in voltage. And yes - a less powerful transformer can be use. I am typically using 150w transformers for the average job.
Also with LED you can have nice colors as well as white!
Regarding white (per your comment) typically you are selecting either warm white or cool white.
LED's are measured by temperature in Kelvins. The higher the temp. the bluer the shade of white. When you purchase your bulbs just check the temp rating or usually the package will tell you which white it is.
Regarding colors.....yes you can buy different colours but what I have found the best is to buy "color changing" bulbs so you can change things up if you want. Some of these color changing bulbs also have 'white' as an option so you only have to buy 1 bulb.
Regarding your situation....I'm not sure the aspects of your tortoises in how it impacts the design but anything that shows wiring will be unsightly. So what if you buried the wires deeper than they typically dig? And maybe in PVC conduit perhaps?
Regarding fixtures...to get the widest beam a PAR 36 bulb would be the one to use. In a well light...like this:
You can buy these Par 36 bulbs in LED - I posted a thread a while back about the color-changing ones and they are indeed very cool.
You could hook up a couple and play with spacing before doing anything more permanent. You may end up with only 3 total lights. Sometimes less is more.
So.....150v transformer, daisy chain with 12 gauge cable, and LED well lights and you're done.
Feel free to reply with questions or comments - be happy to respond.
I'll post that thread of mine next email.