It makes sense - look for "low voltage LED Lighting" to see what's available, or slide on over to Electronics to build your own.
Dimmable LED bulbs are those with drivers built-in that are "dimmable" - if just running controlled current to LEDs, they are all "Dimmable" (lower current, lower light) so, no, you don't want an LED described as "Dimmable" (nor one described as 12V, probably - a "bare" LED with no other parts attached is best when you are going to feed it controlled current.) If you purchase a "12V dimmer" pre-made, follow the dimmer-maker's suggestions on types of LEDs to attach to it.
LEDs are current devices, so controlling the current will give highly consistent light output (controlling the voltage, less so)
First of all, when you read about a "50W LED", it is certainly saying that it outputs the equivalent light of a 50W incandescent light (although that is often a exaggeration). That "50W" LED actually draws less than 10 watts. This means that it draws much less current and emits much less heat, both of which are the factors that typically limit the allowable light in a fixture.
You'd be able to replace a 50/100/150W 3-way with a lower wattage, even if staying with incandescent.
Second, unless the LED light is marked "dimmable", don't try to use it on a dimmer. You may damage the dimmer, light, or both. Even if the light is dimmable, some dimmers will not work well with a low load: a dimmer that works on a 100W light bulb may not work on an equivalent-light-output 13W LED. They make LED/CFL specific dimmers that may work better.
As for the 130V R40, that is probably a "long life" bulb. Running an incandescent at lower than rated voltage extends its life. Therefore, they make 130V bulbs to run longer on 120V. Just use LEDs rated for 120V.
As for your existing florescent lighting, I assume you're talking about the straight tube type. For a fixture that old, it's probably less efficient that newer florescent fixtures or LEDs but still much more efficient than incandescent. This is harder to replace with LEDs, just due to the area covered. There are LED tubes made to replace florescent tubes but they are still expensive and generally require re-wiring the fixture. These are most often used in commercial applications.
Best Answer
A housing can either take a bulb or a retrofit LED ring that sits on the end. If you use a bulb (LED or otherwise) then the angled housing can be of benefit, by keeping the beam pointed in the downward direction. However if you're using a flat LED ring, there's no benefit to using an angled housing, because it's going to sit on the slope anyhow.
If you're concerned about lighting the area in a downward fashion, you can either use an angled housing:
with a LED bulb in it:
or use a standard housing:
with what's called a "gimbal" fixture. A gimbal fixture looks like this:
And will allow you to compensate for the slope in the ceiling, by pivoting the fixture inside the housing.