Are DACS necessary?

I am not a seasoned audiophile, but doing my best to put together a system that sounds great- on a budget. I have an Oppo BDP-83 Blu-ray player, a Marantz SR7000 receiver, a VortexBox Appliance for streaming FLAC files to a Logitech Duet, which outputs to the Marantz. I am looking into a DAC to increase sound quality from digital files, but have also started thinking about, down the road, investing in a good pre-amp and separate amplifier. If I buy a DAC now, will it eventually be redundant if I get a good pre-amp? Is there any reason why a good quality pre-amp would not include a high quality DAC? (Thinking along the lines of Marantz or perhaps Integra separates). Knowing my budget overall is very limited, I am not shooting for the stars here- just solid sound to my infant ears. I would hate to buy a DAC now only to find I have no need for it next year. Any thoughts or suggestions are most appreciated. Thank you.
Bsoz - I'm using Benchmark DAC1 as a pre since I don't have analog sources - otherwise I would find DAC with preamp like Benchmark DAC1 PRE. Using DAC as a preamp eliminates one set of interconnects.
An outboard DAC will very probably improve audio playback from your Oppo. It will not cramp your style if you move to conventional separates later. When you did, you would only have to resell your DAC if you specially wanted to use an (unusual) preamp with a built-in DAC, or a DAC-with-preamp like the Benchmark or the (discontinued) Apogee Mini-DAC.

You will have to factor a good digital cable into the purchase. Make sure it is 1.5m long.

The BDP-83 has a good built-in DAC. The SE version is said to be particularly good with audio playback. If you had the SE I would suspect you needed to get quite a good (i.e. expensive) DAC to better the built-in one. With the standard 83, perhaps you'd need to spend less to get a nice improvement.

I've been using a Muse non-oversampling DAC with my Oppo 981. It makes a nice improvement and was not expensive. The 981 is not the BDP-83 but on the basis of my experience I suspect that even one of the lower-priced NOS DACs like the Valab might do something nice for you.
You can also get your Oppo's DAC modded with the other usual circuit improvements and superclock to better your sound from a one box solution. Again it will most likely not interfere with conventional preamps which normally don't have a built in DACs.
You may also want to check out the Peachtree Nova which is an intrgrateed amp and dac all in one and priced fairly reasonably as well
In a 2nd system I run a Carver Amp, A hybrid Van Alstine Pre-amp and Sherwood NewCastle CD deck. The speakers are Salk SongTowers. I like this combination real well. Within in the week, added a Musical Fidelity V-DAC, three hundred bucks from Audio Advisor. It is a simple unit, easy to wire in, and the difference in sound is immediate and impressive. If you want to try DAC without spending a fortune, this is the way to go.
Regarding "why a good quality preamp would not include a high quality DAC" -

While I've heard some very good preamps and integrated units with built in DACs, they turn me off a little. Sources come and go. It seems like what was cutting edge not too long ago is now obsolete.

The Naim Uniti sounds fantastic. At $4k, its a few hundred more than the Nait 5i and CD5i, and has a few other things too - DAC, tuner, internet tuner, and music streamer. The problem to me is that most of that stuff will be obsolete, and end up being like the 8-Track player/receiver combos. The integrated amp section will stand the test of time, but what about the CD player? Wireless streamer? DAC? In 10 years, that $4k receiver will most likely be an integrated amp with a bunch of stuff that isn't used (because its all been improved on).

99% of the people here probably wouldn't own any piece of gear that long. I guess I'm the exception. Naim gear isn't disposable gear. There's decades old stuff still going strong. It's a great sounding unit, probably far better than any $4k combo of all that gear that I could put together seperately.

There really haven't been very many ground breaking changes to amplification in a few decades. Things like ICE power, etc. have changed things a little,but they're still no where near the mainstream hifi. IMO amplification is the safest investment in hifi. Sources change far too often.

Just some food for thought. I'm sure others' opinions will vary.