Some great advice above. More recent versions cost more; but 2.5 wasn't really very different than 2.0 so that upgrade is often not worth it. That said, the seats are a little different so you might try sitting in both to see what you like. And the front fascia is different, so decide what you like best. (If you like the 2.5 front end, you might be able to find a cheaper "2.25", which is really a 2.0 with just the front end changed). 2.0 did have a lot of small improvements over 1.5 so most people see that upgrade as worth it, but then again a few little things got lost: physical shifter than makes parallel parking slightly easier, carbon fiber bits inside, etc. There are other threads detailing the exact changes between versions. When you're driving, most of them aren't very important.
You said the Roadster will not be primary transportation. If you are going to track it, you might appreciate the extra oomph from the Sport version. But if you're not, given that you are in Denver you might not have racing tires on most of the year, as Rodolfo says that's a lot of money to pay for small gain...that you may not see at all if you change the tires.
If you are tracking, the adjustable suspension will be valuable to get rid of understeer. It can also be set for comfort to make the ride nicer, but if this is not primary transportation that probably doesn't matter.
The base stereo is pretty lousy. But then, the Roadster is very noisy, especially with the top off - and the upgraded stereo ain't so hot either. So you may not care.
The car looks better with the hard top on; but it just takes too long to take it off, and you have to decide before you leave the house in the morning because there's no place to store it on the car. I do use a hard top for the worst winter months, but most of the year I prefer the soft top - rolled up in the trunk whenever possible. It's no big deal using the soft top in winter; weight, noise and heat retention aren't very different.
Private Roadsters (probably overpriced but very clean sample
HERE) are generally a lot cheaper than Tesla's certified used ones. Tesla is including a 37-month extended warranty now, though.
I don't think any of the options are very important; I got a stripper model and I'm happy with it. The options that are important to me didn't come from Tesla: a mount for my cell for navigation; HID lamps; hitch for a bike rack, OVMS.
Tesla quoted me $9k to replace the PEM and $36k to replace the battery. Not that I've needed to; I was just curious. Odd that Jackyche was quoted more. (My figures don't include tax).
It is a blast to drive; unless I'm going on a road trip, I much prefer it to the Model S. It's noisy, it's small, it has a rough ride, it's expensive...but if you are OK with all of those (they are pretty standard sports-car qualities), I can't think of anything I'd rather have.