The recommendations of specific parts or hardware necessities for an organization varies greatly based on the kind of work they are doing and the demands they are expecting from their computer systems and network. This is always going to be the case, so for us to just quickly list off recommended servers for you based on a description of what you are currently running will be quite limiting. It truly takes someone who knows your system, your software, your business, to make a true educated recommendation on your computer system. In fact often times when I am consulting with a customer about a new server system or network equipment, it may take several meetings and up to a month of time researching their current system at all levels before I can even make a proper recommendation.
That being said, there are some things that I might be able to help with to at least get you someplace to start.
Personally I prefer the HP server systems over Dell. There are good sides to both, and I have to say my "allegiance" to HP server systems is definitely torn with the newer Gen8 servers which require HP hard drives and SmartDrive caddies which increases the purchase cost of a server tremendously over using other 3rd party hard drives including Western Digital RE4 or other brand SATA/SAS drives.
We just set up a new ML350p G8 server for a customer and I really liked the system. Everything we needed plus plenty of room to grow. Additionally, there is plenty of helpful information online, and if I needed it an HP rep was there on the phone any time I needed to call and get some answers to any questions. But the set up was so smooth it was never needed. There really was not set up actually. Turn the system on and start configuring RAID arrays in the full-featured GUI interface and then on to the OS installation.
Running your SQL database is probably the most demanding task that your server will need to perform. With this workload, it's my understanding that having a faster storage subsystem (your hard drives) is going to give you the greatest performance and efficiency. This means that you should be looking into fast 10k or 15k SAS drives in RAID 1 or RAID 10 for fault tolerance and performance.
You should consider leveraging virtualization to somewhat compartmentalize your different services on the server. It will require a little more horsepower, but it is also much MUCH easier to migrate in the event you need to upgrade or replace servers. You simply have to copy the VHD and paste it on the new computer running Server 2008 R2/Server 2012 and start up the virtual machine with that VHD attached. This can decrease downtime from a server outage from a week or more to under an hour in some cases.
Don't underestimate your server to begin with and end up getting "just enough" to do what you need. You will find out a year down the road that you could benefit from "just a little more" and have to look at upgrading and possibly redoing a lot of work that could have easily been done from the beginning with a little forethought. I've run into this several times with customers before. They have $X to spend and decide they'd like to save a bit so they step down on their storage capacity to just slightly more than what they are currently using. A year later they are almost out of space and have to replace their drives and transfer all the data again, increasing the cost of that server yet again.
I personally do not know a whole lot about the SQL database systems and the Exchange server either, I've never had an opportunity or need to work on them so far. Hopefully others will be able to pitch in their input to help out here as well!