Road racing or Gravel and Quality level?

If discs are well set up they're not noisy (or no more noisy than rim brakes in wet)

Hydro discs are much easier to set up and maintain than mechanical (cable operated) discs but more expensive.

I commuted on a rim brake bike through all weathers for years with no issues - the only thing I did was to fit auxiliary levers so I could brake from the top of the bars (so more head up in traffic) - they're often called crosstops or inline levers. 

Good rim brakes (Ultegra or 105 level) are hugely impressive now, even in the wet. You just end up trashing your wheels in winter/wet

My tourer has discs but I went for rims on my "fast" bike because I prefer them, don't brake so often, and they're lighter. Also only tends to be used in the dry.

You are however pretty much limited to 28mm tyres if you want to run mudguards and rim brakes. If you want wider for comfort then go with discs. 

You may not be aware but there are a huge number of good steel manufacturers these days - Fairlight, Mason, Shand, Bowman, Genesis spring to mind. Modern steel bikes can be as fast as or as comfortable as you want.

PDK is a typo - probably thinking of PDW who make some lovely metal mudguards.

Yeah, I've done my time with clip on guards, and whilst I'm happy to fiddle with rim brakes for the sake of quieter braking, I'm not happy constantly fiddling with clip on guards, go fixed from the word go! 

Kinesis 4S covers pretty much all the bases above (except the upright riding position, it's a bit racier than my usual bikes but nothing that can't be sorted with an upright stem or even some riser drop bars like the Specialized Hovers). You can even pick a side in the rim vs disc debate as the 4S takes both 

There's always a different view -

I think disc brakes are the way to go for commuting (notwithstanding the noise in the wet) unless your commute is on open roads.  If you're in amongst traffic then the predictability of the braking is welcome as are the lack of grey filth all over your bike and no rim wear.

I have a gravel bike for commuting, although I'm frankly not really sure what makes it a gravel bike rather than a solid winter bike.  It is more upright that my regualr road bike, and has fittings for a rack (don't leave home without one) and mudguards.  It's an Orro Terra and all 105.

It's taken a battering so far and done well - average 150 miles/week

You don't say what your commute involves, but the more upright seating position you get from a gravel or endurance bike makes it easier to maintain good all round visibility.

Decent mudguards (eg PDK) don't need mounting studs, just make sure the frame has clearance.

As another commenter said, decent rim brakes are adequate for commuting, unless you ride like an idiot.

I would probably leave space in the budget for a spare set of wheels, nothing fancy, but having the ability to easily swap between 25mm slicks for effortless summer riding, and 32mm treads for rainy commutes is an advantage.

Get a bike frame which can take upto 40mm tyres.  I suppose this would be classed as a 'all terrain' or gravel bike.  With careful selection of either a complete bike or frame you can have a safe winter and fast summer bike, because reality is that a gravel bike with narrow tyres is just as fast as a pure road bike, when fitted with the right components

Coughs-

Kinesis Tripster ATR V2

As lovely as an out-and-out race bike is, mine is now my least ridden bike (already on the turbo trainer for the winter) as they're just not as practical as a gravel bike (or in my case a singlespeed Charge Plug which is effectively a gravel bike e.g. wider tyres, space for mudguards etc).

Also agree with kil0ran, space for wider tyres is great, I run 32s on my day-to-day/winter bike.

I'm in the minority it seems of people who expressly avoid discs, I just can't hack the noise they seem to make in cold wet conditions (i.e. my commute for 6 months of the year...) so I prefer rim brakes, but then, I have the tools and knowledge to work on them and the experience to know how to ride safely in the wet with rim brakes (brake earlier...) Perhaps discs are a better choice if you're not a happy mechanic!

I'd suggest a gravel bike, ideally on sale, and spend some decent money on mudguards (PDW are pricey but look great and are near silent) and also good clothing, although that's a whole other thread!

Tiagra is the sweet spot for commuting - basically 10-speed 105 and there's very little point in 11-speed unless you're racing. Even Sora will do the job for a commute of that distance. Just upgrade the brakes to 105 level if you go with rim brakes.

As far as frame goes make sure it has mudguard eyes, clearance for at least 28mm tyres, and ideally eyelets to fit a pannier.

Those requirements are met more easily by a disc-braked frame than rim brakes but discs will cost more and weigh more. If you're commuting in foul weather though I'd recommend at disc frame, or a set of winter wheels.

Most importantly go to your LBS and chuck your leg over a few to get an idea for frame sizing as it's easy to end up with something uncomfortable. Get them to show you the difference between a head-down pure race machine, an endurance bike, and a crosser/gravel bike.

We're just entering the end of season sales so it's a perfect time to buy, but if it's your first road bike you really need to try before you buy so that rules out online sales. 

There’s so much crossover between types of bike these days I’m not too sure how much it matters. 

The real key for me is that you love it, no matter what it is. From the paint job to the ride to the correct frame size (massively important). There are always tweaks you can make to adjust for the seasons/usage.

Getting up in the morning with a smile on your face because the next hour is riding a bike you love is priceless.

Lots of bargains to be had out there - good luck!