What I would do is to get a copy of the car's logs. This can be done with a simple USB memory stick (4gb or less). Put a directory "VehicleLogs" (case sensitive) in the root of the drive, and stick it in the USB port that's just above the 12v Accessory outlet at the front under the center console. It should start blinking, and give you a 3-beeps warning "memory transfer in progress". Takes about 15 minutes or so. The car can be plugged in during this time, but cannot be actively charging.
There are viewing applications available (search for "VMSParser.exe") that will let you know what the health of the battery is, and as important, any diagnostic alerts the car has coughed up that might indicate troubles that might not be immediately apparent during a test drive. Be on the lookout for 1144 / 1146, for example. The battery's health is measured mostly by its CAC. Brand new is about 160. My 2010's original battery is about 139 now, and I charge to 164-ish miles in standard mode, 214 in Range mode. That's apparently about average for the car these days. I have 49k miles on mine. Note that the Roadster's logs have a lot more information in them, and can be interesting to browse for past, um, indiscretions. My car, apparently, hit over 100mph at one point prior to my ownership... On the track. Of course.
Note that the Roadster's terminology for modes and range (ideal vs estimated) are different than what Tesla now uses for the S/X/3. You want to know the Ideal range, which is only based on battery health, in either standard or range mode. The Roadster "hides" the top and bottom 10%-ish of the battery from its display in Standard mode, vs Range mode, so the number displayed has to be taken in that context. You also charge in Standard or Range mode, independently of what you drive in. Very confusing to explain, but you'll understand once you start using it. I agree that 90 miles of Ideal range when the battery shows 3/4 "full" is a concern, but not a showstopper. Get the logs to know the true story.
To the question of unobtanium parts, that is definitely something to be concerned about long term. Many of the mechanical parts - brakes, tires, etc., especially the wear items - are easy to obtain, and if not the originals, equivalent or even improved substitutes can be had. The electronic parts, however, can be difficult and/or expensive to replace. There are weak spots in the car's design (as in any car), notably the connector that drives the PEM / Motor fan assembly (failure of which relate to the 1144 and 1146 errors I mentioned above), and for some, the eventual breakdown of the insulators under the IGBTs that drive the motor. Depending on your skills, these can be repaired, but the default Tesla repair is in the $10k range. The good news is that Tesla does still service the car, even out of warranty. It just might take a while, or be expensive. Being in California probably helps.
Not trying to scare you, by the way, but just to be aware. This is a hand-crafted, limited-production prototype vehicle. It's an incredible car to drive, and an historic piece of modern automotive history that is an honor to own. Mine is my daily driver, and I wouldn't have it any other way. But every car has its faults. I used to own a 1972 Datsun 240Z, and it had me swearing in Japanese on more than one occasion due to its electrical system problems. There are fewer things to go wrong on the Roadster, but they tend to be bigger deals when they do.
