While I'm not sure if any PPC PE device can be used as a fax modem to be
used by an external fax, they surely can't be used as faxes themselves
because in WM5 HTC models it's been completely disabled, as has also been
pointed out in several of my articles & posts. This is why for example KSE
Truefax you've mentioned (or ANY Pocket PC fax app) won't work on your MDA.

Maybe you can investigate one of the services that will fax an
electronic document for you - you send them the document over regular
data connection.  I do not know what kind of volume of faxing you do,
but such a service when used in conjunction with VOIP might still be
cheaper than using a standard landline setup.  I believe, and this is
subject to change, you would need a fairly low volume of faxing for it
to be cost effective.

Some VoIP is- a few providers use a different codec for fax (but
generally charge you extra for a separate "fax line" to enable it.)

Having said that, it's still not as reliable as a POTS line for faxing.

Short answer, no.

Long answer, still no, but here's why...  ;-)

Windows Mobile fax software like TrueFax allow you to fax documents on
the device to a fax machine via a cellular call.  Several problems with
this- first, there's
no way to connect a fax machine to the MDA, second, the MDA, IIRC, has no
fax capability, and lastly T-Mobile doesn't support fax
calls anyway unless you pay $10/month extra for a "business CSD" add-on
line to your account.  (I'm assuming you're on T-Mo since you have an
MDA.)  I'll spare you the technical details of why the special fax
service from T-Mo is needed- suffice it to say that digital cellular
handles faxes even worse than VoIP does.

Realistically, your cheapest and most reliable solution is probably to
use an e- mail-to-fax service like Fax1.com, jfax, efax, etc.
Essentially you e- mail a document (computer generated or scanned) to
(destination-
phonenumber)@fax1.com (or which ever service you use) with your
credentials (username/password) in the subject line and they fax it for
you for a fee.  Some providers charge a monthly fee which includes x # of
faxes, some charge a fee per page, some charge both.  The best deal
depends on your monthly faxing load.

I use fax1.com because they have no monthly fee, and I use it exclusively
for faxing "on the road" from my MDA (I use it just a few times a year,
so I accept their slightly higher than their competitor's per-page rate
in lieu of paying a monthly fee for a seldom used service.)

Most e-mail-to-fax providers also offer fax-to-e-mail as well, where they
can assign you a fax number to use for incoming faxes which they convert
to .pdfs or .tifs and e-mail them to you.  
Fax1 charges too much for that (for my meager needs) so I use
Callwave.com's "Faxwave" service for incoming faxes.  If you don't mind a
rural Iowa fax number, freedigits.com will give you free fax-to-e-mail
(incoming only.)  

Good luck!