Roll my own, or hosted?

Ok - apologies in advance for this noob inquiry.

My situation:
I’m a windows guy - but I also supports macs and am minimally functional in *nix. I can type man man.

I’ve been glancing at voip solutions periodically. I know we’ll need something like this eventually, but I need a little help deciding if today is the day.

Our company has multiple small locations in three major US cities.

We’d like to have better call handling than our current pots lines allow.

I’ve looked at hosted systems that run around $250 a month for 5 lines.
These include all the features I’m looking for, including the ability to change ivr and call routing on the fly via a web interface.

I am the full time IT guy, so we can tolerate a bit more local administration of the system than a company with no IT staff.

Our most line intensive location right now has 4 lines, but really could use 6. One other location is similar, and the third major location will soon be coming on line with similar requirements.

We have some call center-like needs - we have a daily Call List serviced by three people who spend half the day making outbound calls to existing customers to get their weekly orders.

At our level, the price for hosted would be a wash - equal to our existing recurring costs. So, I’d be offsetting the substantial equipment costs against the increased functionality/productivity.

OTOH - if I can roll my own and just get the 5 sip trunks for $100/mo or so, with unlimited in/out and nat ld - I can obviously amortize the costs much more quickly.

Ultimately, I’d like to have a system with an ip pbx supporting at least the three main locations. I want users to be able to click dial from Outlook, and preferably from other apps as well - eg: excel sheets.

I’d need to be able to support day to day activity and most changes myself, and the office staff would need to be able to change call routing and ivr config through a simple gui.

Would it make sense to set up asterix with the existing pots lines, add new lines as sip trunks, then port out the pots to sip after the system has proven itself?

Will I be able to get the gui I need without going with a full commercial program?

I’m not averse to paying some upfront costs to get the thing set up, but
A lot of the guides are pretty dated, so I am asking the experts whether I’m biting off more than I can chew if I roll my own.