Reddit mentions of Supermicro SuperServer E300-8D - Mini-1U - Xeon D-1518 2.GHz - 0 MB - 0 G

Perhaps the next generation, but for now the max you can get is 16GB

Ways to get around this limitation:

• Don't run those images on your Mac, rather host them on the cloud using a service such as AWS or Azure. Connect using Remote Desktop. This options cost a couple of $ per month, but has some inherent advantages such as being able to share with team, 24x7 always on, backup in case of laptop loss etc.
  
• Run those windows instances on another dedicated server on your local network, and connect using Remote Desktop from your MacBook. For example, I have a nice Intel Xeon D-1518 running headless in my basement, and a pfSense VPN appliance that allows me to connect from remote locations. Disadvantage is that you have to buy the hardware and set it up, but once done the only cost is electricity, and virtualization software.
  
• Do you actually really need those heavy Windows virtual machines? Are there alternatives such as Docker images that can do the same job with a lot less operating system overhead? Eg. instead of running a LAMP stack on Windows, run it on a Linux based Docker image? You might be able to squeeze into 16GB with a bit of housekeeping.

How about an i5 or i7 Intel NUC? Doesn't take up a lot of space, 32GB of RAM, and generally quiet.

SuperMicro has some small Mini-ITX barebones boxes that have multiple NICs, support 32GB of RAM+, and Xeon processors. https://www.amazon.com/Supermicro-SYS-E300-8D-Intel-D-1518-Server/dp/B01M0VTV3E

https://www.amazon.com/Supermicro-SYS-E300-8D-Intel-D-1518-Server/dp/B01M0VTV3E/ref=sr_1_5?ie=UTF8&qid=1535071234&sr=8-5&keywords=xeon+d

Need to add memory and a disk, but solid piece of gear in a case. Also the power supple has a screw in connection which is kind of nice.

You can always go the micro server route:

https://www.amazon.com/dp/B01M0VTV3E

Board specs:

https://www.supermicro.com/products/motherboard/Xeon/D/X10SDV-TP8F.cfm

35w cpu, 8 thread processor, can take up to 64-128GB ram depending on type, has 6 1G ports and 2 10G SFP+ ports, USB 3, has m.2 support and space for one 2.5in drive.

A bit pricery than the above, but way more flexible/powerful while still under $1000 kitted out. The built in 10G makes it easier to go to proxmox hyper-converged in the future if you want to scale up with ceph to boot.

Honestly with your use case you could also look at Dell T30 and similar class tower servers, there was a sale 2 days ago at which you could get one with Xeon for $299. Offers much stronger CPU, DDR4 RAM support (also with ECC) up to 64GB etc. Will cost more upfront but E3-1225v5 is 2-3x faster than Celeron G.

Another option would be a Xeon D server. Supermicro has a whole family of them ranging from 2 to 16 cores, 1U versions or boards with CPUs sold separately (standard desktop mITX or mATX size). They all support up to 128GB RAM, 6 SATA drives and give you 2x 10Gb NICs (and that alone is worth like $300), KVM IPMI and so on, all at really low TDP. Asrock also builds these - their versions are not so well equipped as you don't get 10Gb and 4 SATAs are accessible via miniSAS adapter but they can be found at a really decent pricepoints when bought used. For instance I just grabbed one (I live in Europe though) for 289€ including shipping (and finding a new hardware with enterprise level stuff at this pricetag is a REALLY good deal). It's a standard mITX size so you can use a typical desktop chassis and PSU. Of course this would still mean having to add drives and RAM but if you have heavy virtualization in mind then having a Quad Core CPU + ability to shove in 32+GB RAM is very nice to have.