Bib shorts: Raffa basic, Castelli mid or DHB top level?

I find Ale with the 4 hour pads my go to shorts but being a good deal bigger than you I have to get xxl.
Can't see what you're going to stain them with apart from sweat as the straps would sit underneath your jersey.
As others have said pad is the deciding factor and it took me a while to identify my favourites.

Rapha Core are my personal pick at the moment, I can't comment on sizing though unfortunately it just works for me. I currently use Santini, Rapha, Castelli, Vulpine and Alpine CC bibs and the Rapha Core's are y favourite at the moment! I tried the Pro Team's but much preferred the pad and grippers of the cores.

You mention "short, chunky build". Given that this is the case (particularly if your thighs are more on the sprinter side, rather than climber), I would probably discount the Castelli, and go for the Rapha.

My legs are on the short and, erm, well-built side, and I found that I have had to size up on Castelli shorts to the extent of taking XLs, at which point the legs are too long (at least for my taste). I generally take L or M in other shorts sizes (e.g. Assos). I am about 177cm and 73kg in wieght.

I presume your weight is 74kg, rather than 174 as you mention above (though if not, then chapeau for taking up cycling!).

I tried on various Rapha shorts in the recent sale - the cut varied a bit, and I didn't try the Core shorts, but I would guess that the Core shorts, like the Classics, are likely to be on the more generous side in terms of fit (I needed to go to L with the Pro Team ones). I reckon an M size would probably be good. I found the M size Classic shorts slightly too big, but I have been cycling a while and slimmed down a fair bit. I reckon they'd have been a great fit 5 years ago or so when I was still getting started.

One final tip - the version with white straps will go better with the lighter-weight jerseys you will be using in hot weather.

I'm 175cms and 65kg, as I said Castelli medium fits me perfectly.

I'm guessing you meant 74kg and not 174kg!

In Castelli, at 1m82 tall and about 80kg I have been an XL or XXL in the past so be warned about only upsizing one jump. In Rapha I'm an L.

I've never tried the Rapha core shorts but I've got 3 other pairs of Rapha bibs and they're the first pick for me every time. Can't really go wrong. 

Rapha Core would be my pick. Medium in them will be fine. Wouldn't go large with the Rapha Core's as I'm guessing you're planning on getting fitter. The Core Rapha range is surprisingly stylish and good quality. It's pretty fitted actually. 

I would wholeheartedly recommend the Rapha core shorts. They are the best shorts I've owned, even better than Rapha's more expensive ones. Snug and supportive, excellent pad and grippers, good sensible sizing. I find Castelli sizing way too small. I am a lady btw but I would have no reason to think the men's Rapha would be any less good than the women's. Haven't tried DHB Aeron so can't comment, but I hear good things about them.

I can only speak for the lower end DHB and high end Castelli products.

For me it's been all about the pad, I had some cheapish DHB shorts and was fairly unimpressed with the pad.  I bought some mid range Castelli bibs after that which were much better, but then I got the high end ones with the Progetto pad which were a lot more comfortable yet again.

I seem to be a ringer for whomever Castelli model their "medium" sizing on, I've got a load of their kit in medium and it all fits perfectly - so I've not had sizing worries.  My current bibs of choice are Castelli Free Aero Race Bib Shorts - generally you will be able to get them for £100ish discounted.

So not a lot of help, but a couple of things to consider:

- bibs are way more comfortable than shorts IMO

- try and get the best pad you can for the money, it's the most important thing