How to convert a commuter CX bike to an Electric bike? I would like to convert my current bicycle into an E-bike. 
While there are many conversion kits available and plenty of sites with tips on this, none of them quite meet my needs, or they fail to explain exactly why certain choices have been  made.
I am looking for an answer that provides insight into exactly what matters in this type of conversion. 

Current situation:

I commute to work on my bike 2 times a week, which is 75Km (45 miles) round trip. My bike is a 2010 aluminium cyclocross bike with 700 x28 road tires and standard canti brakes, otherwise Shimano Ultegra parts. Front fork is carbon. The terrain is flat, with about 5% gravel roads. Location is Northern Europe, so plenty of rain.
I normally ride this distance with an average speed of 26 - 28 Km/hour (16 - 17.5 M/h), depending on wind and traffic. 

Budget for converting is not very high (few hundred $, certainly below 800$), but I am willing to spend quite a lot of time on sourcing the right parts and am pretty confident in my DIY skills, including building wheels and batteries if need be. Getting another bike to commute on is not an option. 

Reason for converting

Main reason for wanting to go electric is reducing travel time. 
The two commutes a week per bike are a part of my exercise schedule, so reducing tiredness or getting to work easier/without sweating are not important. Ideally I would like to convert and still get a reasonable amount of exercise, only get to work faster (35-40 Km/h)/(22-25 M/h). This means that I still put in the equal amount of energy that I currently do (meaning I can reach 28Km/h on my own) and the motor supplies the extra energy required to go 35 Km/h. 
 In case of a removable battery, charging at work would be an option, but an effective range of 75Km to be able to make the round trip without charging would definitely be very nice.

Decisions to make 

Motor type and placement: What I understand so far is that within my budget, a direct drive 48v hub motor would be best. I do not really care about their added weight or drag when not active. For weight distribution, a front wheel motor would probably be nice, but I am not sure how well this goes along with my carbon front fork, and if it even fits (over locknut distance). When looking on e-bike motor sites, almost all of the motors seem to be very high power (500W/1000W). I really doubt whether I need this amount of power, when all I need is a little support to get from 28 Km/h to 35 Km/h. I do not expect the bike to pull me along without myself delivering most of the effort. Would something like 200W not be enough?

Choosing a controller:Controller should be compatible with the motor and battery, and not be limited in speed to at least 50Km/h. Regenerative braking would be nice, mounting to a drop-handlebar is nice, but I expect to have to make some custom solution for this anyway. 

Choosing/making a battery:
Obviously, some form of lithium ion battery is best for performance/weight. But what are other (budget friendly) alternatives? Is a sealed lead battery really such a bad idea?
How much costs savings can I expect when building my own battery out of Lithium cells? and how feasible is it to import them from China? Battery mounting will probably be some form of custom rack.

Getting second hand parts:
What are my options regarding second hand parts, what is the life expectancy of an e-bike system (excluding battery, of course)? Is getting a second hand donor bike for electric parts a viable option (they are often found quite cheap here, when the battery has died they are sold for 300$) It is of course difficult to find exactly the parts (mostly motor type) you need in a second hand bike.

Question

Is there anyone with experience with such an conversion who can provide some insight into what really matters to make a fast bike go even faster with a custom DIY solution, without me having to empty my bank account?
I live in the southern most part of Nashville, TN (quite hilly with a major hill between my community and the rest of Nashville). I ran into @Blam's point exactly and have been considering investigating this new technology.

http://senseable.mit.edu/copenhagenwheel/index.html

The weight is the lowest comparable to other options (battery variable).
TOP SPEED   US: 20 mph, EU: 25 km/h
BRAKE TYPE  Rim brake and regenerative braking (downhill and back-pedal)
WEIGHT  5.9 kg / 13 lbs
DROPOUT 120 mm (single speed), 135 mm (single speed and multi-speed)

I haven't ridden one yet but it may be worth investigating.