I'm only adding to this thread because I'm super picky about fit and comfort and have a closet full of exposure protection I wish I hadn't bought after quite a bit of trial and error over the years.
The above post by Force-E is spot on. I'm in a warm state but dive do dive in cold areas often. I have mail ordered a bunch of suits over time because none of the LDS stock more that 5mm and very few if they did. Last one I bought, I happened to be in San Diego so they had a bunch of thicker suits in stock. Tried on 12 7mm suits that day and walked out with what is for me an great fit and easy to don 7mm semi dry. The shop operators where bringing me water, commenting that most people either buy or walk after about the second or third try on of a thick suit. I ended up with a brand I would have never considered, also one of the more pricey selections. After trying everything on I tried the one I decided to buy after making a mental note earlier and it was a no brainer when I tried it on the second time after all the others, try several on and you will know when you get that one that fits.
- If it's hard to put on in the shop, it will be worse on site
- If it is uncomfy anywhere after about a minute or two pass on the suit
- Manufacturer size charts are all over the place, don't trust them, you need to try it on.
- Forget about being brand loyal, forget the hype and gimics, not nearly as important as a good fit.
- If a cold water suit then go for good neck ankle and wrist seals to minimize water transfer.
- If you buy by mail/online you absolutely need to be able to return, you will eat the shipping cost but that is just part of doing business. There is no way a mail/online store could not require this without losing their arse, it is a service worth paying for.
- Definitely some merit to custom, and may actually be quicker and less expensive after all is said and done. But, you should know what you need before you order.
- In thinner suits at temps where compression isn't an issue the newer strech neo suits are very forgiving if the size isn't perfect.
- Once you get below about 65 degrees (for me personally) all the gimics in the expensive high end wetsuits cost so much that you should probably consider a $150 drysuit course and maybe an entry level drysuit, by the time you pay for all those price neo layers you can dive dry below 70, once you do you won't go back
Since your new, you may want to just get a 3mm and a beanie which will always be useful in your diving as you move on. Then maybe rent/borrow a few suits to figure out what layering works for what you are doing. Then proceed to buy once you dial that in. It really is a subjective matter and no two people are the same regarding hot/cold natured, fit, etc.
I've been diving for a long time and the number one sore spot for me regarding gear has always been exposure protection. I finally got it all dialed in now and I"m happy with what I have. No need to say what it is, because it likely won't work for you. It is something you need to experiment with.
Feel free to PM for info.