Greetings @lukejngodley and welcome to expat.com!
I like Plovdiv a lot! It's a former European Capital of Culture, and Bulgaria's second city. I find it far nicer than Sofia, but it depends if you want the bright lights of the big city or not. Plovdiv has a nice mix of old and new, plus it's a good size: large enough to have all facilities, small enough to be easy to get around, and doesn't get completely jammed up with traffic.
We chose an area called Hristo Smirnenski. This about 2km from the main center, so it's very quick by bus (10 minutes) or car/taxi (5 minutes). This area has the Regatta lake (and river) and Youth Hill, so plenty of green space for walking/exercising... but it also has the new Mall Plovdiv and Kaufland/Lidl close to that. There's a shiny new Pulmed hospital here too.
There are also some nice villages just outside Plovdiv if you like a more country scene. Markovo and nearby are the best I think (getting up into the hills, with a nice view of the city).
There is loads of construction going on all over the city and in Markovo. So there are many new developments you could buy before/after completion.
We purchased our initial apartment via an agent, and our new apartment we got direct from the developer, pre-construction. Currently, you can expect apartments in Plovdiv to be priced at about 1,000 euros per m2. Note that the quoted total space includes common areas, and balconies, so it's not your internal usable space.
I believe that Bulgaria's biggest agent is BulgarianProperties. They have lots of listings in every city, and they have listings in English. This site certainly will give you a good idea of what's available in Plovdiv, and at what price. They have lots of English-speaking agents, and are used to (or perhaps focus on) dealing with foreigner buyers. So this is a pretty easy way to buy. I've used them, and they've been very helpful.
The other good site is OLX.BG which is big with locals. The listings are in Bulgarian, so you'll need a browser with automatic translation. This is good for research too, and you'll find quite a few listings direct by owner/developer. I have purchased via this site, but it's a bit more of a challenge to view/buy as not everybody speaks English.
Our developer was HUS, and they've built a number of very nice apartment blocks in Hristo Smirnenski (Plovdiv City Park development). The developers of Panorama Park West and Maritza Gardens are quite established too. I don't think it's cheaper to buy direct (although you might be a much better negotiator than me). But they sell them "Bulgarian standard" without any finishings, so you can get it all done to your exact taste (but you have to factor in the cost of doing this)... and you can pick the best apartments (we grabbed a couple of neighboring top floor apartments with great views).