Apologies if this is teaching you to suck eggs, OP, but if you genuinely know nothing about the housing market or purchase process and are playing with half a million quid, I’d suggest doing a lot more research before you enter into negotiations with anyone (esp a developer).
You need to really get to know your local area and market inside out in order to get the best deals and be ready to move quickly if something good comes up (macro trends in the economy are obviously worth being aware of, but regional and local property markets have their own ecosystems which can go against overall trends). As a very broad rule of thumb it’s much safer to invest in the most affordable (or worst!) house in the most desirable road or area than vice versa.
The best way to get acquainted with the market is to be on top of it every day - everything’s online so it’s super easy. Research sold prices on Rightmove which will give you detailed info about different streets and locations; see which properties have hung around (generally means they’re overpriced, but can also indicate there’s some other issue putting buyers off); check aerial maps (train lines, rubbish dumps, public footpaths); ‘drive’ around the road and area using street view; look on ‘my town’ type forums and at crime stats to see if there are any hidden problems; set your alerts to be notified as soon as there are new listings that meet your criteria.
The most powerful negotiation tool is knowledge. Be better informed than the estate agents; know how long things have been on the market or what any drawbacks might be; know which properties have achieved the best prices and why. The more you know, the quicker you’ll recognise a good deal or see that something’s overpriced and where you might be able to make a cheeky offer. In your shoes I’d be looking up to £550 - you never know what might be possible with a following wind.
Also be aware of your sunk costs and the different thresholds (stamp duty, conveyancing, fees etc) and if you’re considering flats or apartments always check the lease length, ground rent and service charges (these can be a dealbreaker - things get very expensive very quickly in well maintained properties with lifts etc). With developers, check the small print, covenants and warranties, and research their existing projects in case of any post-purchase issues with build quality etc. Good luck!