aliasfox

Might be worth calling around to see if any of your local shops have discounts on leftovers from last year, or even 2018. When I was shopping around last year, I saw a bunch of good deals, from a discounted Colnago C-RS with Ultegra (fast, but didn't agree with me) down to a Cannondale CAAD12 with 105 for $800 (wrong size), and a heavily discounted Bianchi carbon with 105 in between.

This works better if you know what sizes to ask for - I am, depending on the geometry, comfortable down to a 53cm and up to a 55cm. Also, it's worth getting a test ride - at the very least 5-10 minutes around the block, even better if the shop will let you take it out for a 'real' ride. For me, personally, that's when I know I bond with a bike.

If you're looking specifically for something worth upgrading piece by piece over time, I'd probably lean towards something with:
- Disc brakes (they're probably the future)
- Tire clearance >28c - oftentimes, claimed 28c clearance is truly clearance only for 28mm - but as a lot of us know, 28mm tires often balloon out to 31-32mm, which would cause tire rub. My Lynskey claims 28mm clearance, but I have to run a 25mm Conti GP4k because the 28mm tire that I have (a Conti Ultrasport) ballooned to 31.6mm based on my calipers.
- You'll probably want a threaded bottom bracket - it seems like the market is slowly moving away from press fit, which caused creaking issues after extended use
- I would personally look for steel or titanium, but this really depends on how long you plan on keeping your frame. With care, either one can last decades, but I've heard that older (>10yr old) aluminium has a risk of suffering from fatigue, and there always appears to be a bit of concern whenever a carbon frame gets impact damage. Again, this is personal preference, and I'm sure there are countless examples of older aluminum and carbon that's out there every day.