I have a 1992 Korean made Sigma DM1.

I want an accoustic that I can plug in. I very much like this Sigma. Sounds great, like the action, everything.

I'm thinking about getting it wired, but not sure if I should buy one already wired instead.

I have a good guitar tech who can install Fishman Matrix Infinity for about $200 total.

If I were to buy an accoustic electric I'd probably buy a cheap one - not yet willing to spend $700 on a Martin, etc.

Is there any reason why I shouldn't modify my Sigma?
Well. your Sigma is well broken in, and the top is likely sounding as good as it's going to get.

I like factory installs with onboard preamps. The factory takes all the risks of sawing, drilling, routing, etc., and you get immediate rewards.

With add-in systems, you get to pick which type, or even multiple pickup systems, are going in. With that said, these could get very expensive, very quickly.

You do realize that, "a $700.00 Martin", is as cheap as they come. Those are HPL laminate bodies, sort of like Formica.

Electronic and some basic effects can make an average guitar sound a lot better, and plugging in can turn the mediocre into something decent.

OK, single point under saddle pickups are pretty much the rule in you projected price range.

You should probably audition some guitars in you price range before you make any decisions.

These should include the following: Yamaha FGX-700 ($300), Epiphone Dove or Hummingbird, (both $300.00), and especially the Epi EJ-200CE ($400.). A slight price bump upward would allow you to consider the Seagull S-6 with Q-1 electronics (about $550.00).

As far as recommendations go, the Seagull is at the top of the pack overall.

Epiphone's QA has been sketchy in the past, but I just bought an EJ-200CE (LH), which looks, sounds and plays, just fine. These have stereo electronics, offering a lot plug in possibilities. (I believe they're made by Sammick, which is a large OEM, as is Sigma).

Now, the, "check me out list" above, offers a lot different woods, and even a cedar top model, (Seagull S-6), so there should be something there to suit your taste.
Have you heard the Infinity pickup in other guitars similar to yours? That's a lot of money ot invest in a pickup system for a low-end guitar.
My reverbnation page

2012 Taylor 310ce
2011 Fender CD140SCE
Ibanez 12 string a/e
73 Epi 6830E
72 Fender Telecaster
Epi Dot Studio
Epi LP Jr
Chinese Strat clone
Washburn Mandolin
Luna 'tatoo' a/e uke
antique banjolin
Squire J bass
Sure you can mod your Sigma. Many with expensive acoustics install something like a K&K mini piezo SBT (soundboard transducer) pickup and use an external preamp, eg a Red Eye. - Which is what I would do with a piezo. However, my preferred choice is a soundhole pickup. I make my own, and also have a Baggs M1 active with added tone controls in my old L-00. I connect them to the amp/preamp through an endpin jack