I think there are a few issues to consider that we can't address without more information. But just to get started, yeah- if you can sell a long gun to finance a sidearm that would be great. Even a bit more money will bump you up to another class of firearms. The S&W Shield sells for around $399 here and would serve you well. I'm not a big Glock fan but a Glock 19 would be around $500 new and is a good gun. Used Glocks are abundant and a good deal. If you want a revolver you can get a new Ruger LCR for $425, a used for less. The more you can spend the more options you'll have. If your budget is your budget and you can't change it then the Bersa BP9CC is another good option, easy to shoot well and around $300.
How important is size to you? What state do you live in? What are your laws like? People gravitate to different guns based on preference, local laws, weather, etc. For example I live in SD. Open carry is legal here and it's a pro-gun state but I have never seen anyone OC. In the larger towns/cities I think you'd get a lot of strange looks. In the small towns, not so much. I CCW and printing is not illegal. Still, the first C in CCW means concealed! My policy to be discrete.
If you want a very small gun that's easy to conceal a Ruger LCP or Beretta Pico are .380s and about the smallest guns on the market. If you want a wheelgun then a Ruger LCR or Smith J-frame will be pretty small. I often carry a Beretta Nano 9mm. It's small enough to hide under a T-shirt but big enough to still be easy to shoot.
IMO the reliability gap between revolvers and autos isn't that great, if you look at just high quality autos. If you get a quality auto you shouldn't have any issues. Revolvers have their own issues, too (they can get tied up if a bullet jumps crimp, they can be short-stroked, etc).
With autos as a general rule reliability improves with size. A big gun will run better than a tiny one.
When I need something discreet (eg warm weather when I'm lightly dressed or going somewhere where being armed would be frowned upon but legal) I have usually carried my Ruger LCR or Beretta Nano. When it's warmer and I'm wearing long sleeves or a fleece I'll normally carry a full sized gun like my HK P30S, HK VP9 or even a P30L.
Do consider that you'll need to budget for a quality holster (for safety and to enable rapid, reliable access to the gun), a good belt and some ammo for training. If you haven't done much pistol shooting it's a bit trickier to shoot them well vs a rifle or shottie.