All good advice!

Nids can be good with more than just the current "Net darling" I personally run two Broods of Hormagaunts with dang near every list I build.

Either the Wrath, or the Swarm box give terrific value.

Because the Tervigon is of moderate value (currently) you might look at magnetizing and also have a Tyrannofex

It is one of my all time favorites for lower point games. I run it with Acid Spray, and a Thorax hive for a second Template, Electro grubs for Haywire, to mess with pesky armored vehicles.

I like putting Adrenal Glands on for 200 points total. She is almost unkillable in a low point game.

And she can wipe out whole units with amazing ease.
You'll want a Tyrant (with Wings), the next big purchase is the the Venothrope/ Zoanthrope box, if you build one Zoey, and two Venos, you will be happy about 90% of the time.

I like running two Zoeys, but I double the Venos for durability any way.
After Winged Tyrants, the Heavy slot is the strongest part of our Codex, so filling out there is a good idea, each makes a fun project, andadds a lot of table strength.

I personally consider Mawloc to be a super star, but if you have bad luck, it can perform dismally.
If you get the Baal box, it kind of forces how you build, so consider building how you wish instead.

I think the Brood lord mob is possably playable, the rest I just don't see as very good.

The the special rules it grants are dang good, so that might save it.
I normally suggest the Swarm Box, and a Tyrant, that gives you a lot of Tyranids, and you can make a very good 500 to 1000 point force right there. Add a box of Warriors, a Box of 'Thropes and you will have plenty of options.
Once you have a 1000 points built, and have a bunch of games played you can look at adding a Formation to jump up to 1500 to 1850. Living Artillery Node is a very good one, and I personally Love Sky Tyrant.
The "best" Current build for a Carnifex is the same as a Tyrant two twin linked sets of Devourers with Brain Leeches, but that could chang, so magnets are your friend.
In any case, Welcome to the Hive Mind!