Car choice: 1985 Mercedes Benz 300D (turbo diesel, W123 chassis)
Pros: Cheap, durable, lots of replacement parts readily available, pre-chamber engine design is more tolerant of different fuels than direct injection. Lots of people have done conversions on these cars, so lots of info is available.
Cons: US models are always loaded with options, making it more complicated and expensive.

WVO System Design Choices: 

Two tanks vs. Single Tank: Trying to use a single tank is mostly a bad idea from what I can tell. The main reason is that the VO and the engine need to be at operating temperature (180F) before you start introducing VO as fuel. Most people think adding VO before the engine is hot will cause coking and reduce engine life. Two tank design allows most flexibility, avoid engine component modifications, generally considered best choice for maintaining longevity of engine, ensures good start-up in cold weather, easy conversion project. Two tanks also allows a level of redundancy, so if there is a fuel or filter problem, you have a backup plan. Drawbacks to having two tanks is that you have to open trunk to fill up tank, have to add additional fuel gauge, have to add solenoid valves. Notes: If you run the vent tube into a second tank, the main tank will draw in fuel from the second tank because of the vacuum created. Some people suggest skipping the conversion and just mix gasoline with the VO to thin it down. I haven't found evidence that this does or does not shorten the life of the engine, but my gut feel is that it is a bad idea.  

Two solenoid valves: Provide 2 separate 3-way valves, one to switch fuel source, one to switch fuel return. Pros: Allow complete control of fuel path, so that prime and purge can be timed such that VO and diesel do not mix. Cons: You can fill up diesel tank with VO or VO tank with diesel if you forget to switch your valves. (this could solved by using automatic control)  

Use stock lift pump, not electric fuel pumps: Most people agree the stock MB lift pump is plenty strong to pull VO as needed. Pros: Less parts, save money. Cons: Must fix all vacuum leaks in fuel lines, lengthens fuel path of purge needed, requires the use of valves to switch source.  

Fuel Return Design Looped allows gradual transition from diesel to WVO without slug of cold VO hitting IP, but wastes a little diesel to do this. Looped design doesn't allow path for air bubbles to escape. Return design requires additional fuel line run back to tank. I prefer to retain original engine design and use a combination of both designs. (most of the return fuel will be looped, but there will be a small about of return fuel going back to the tank) 

Use separate filters for VO and diesel: Pros: Gives redundancy so that car may be switched back to diesel when VO filter plugs. Cons: Uses more space under the hood. (Can mount the filter in the trunk to save underhood space)  

Hose on Hose or Hose in Hose Flexible fuel hose capable of holding up to VO is expensive and bundling with another coolant hose will be very bulky. Working with metal line is difficult. Nylon or PEX hose can be used instead of metal line. HIH has much better heat transfer than HOH, can probably eliminate FPHX if HIH is used, since HIH acts as a heat exchanger. 

(option) Flat Plate Heat Exchanger (FPHX): Pros: Best way to get heat from coolant into VO fuel. Cons: Takes up more space under hood. 

(option) Electrically Heated Injector supply lines: Pros: Provide final heat after IP. (I've read that trying to raise heat above 180F before the IP is a bad idea, since the IP will absorb the heat, and seals inside the IP don't stand up to the higher temperatures well) Cons: Additional draw on electrical system, additional problem of how to control heat... change in fuel flow will change temperature transfer.  

Fuel Temperature Gauge: Locate temperature sending unit on fuel return before the return solenoid valve to measure temperature of fuel that just went to engine. This will provide a method of determining when diesel has replaced VO on purge cycle. Pros: Ensure that VO is purged while minimizing mixing of diesel to VO tank. Cons: Higher cost and another place for leak to occur. Here is a typical diagram:  

Notes: Danalin Scott says proper VO temp is 200 to 275F, before injector, but avoid temps over 180F before IP, as heat will be lost there. Craig from plantdrive says proper VO temp is under 212F. blackstone-labs.com can analyze motor oil for about $22 always test for polymerization of motor oil when performing an oil change decreased oil change interval is recommended water injection system might help with coking problem make sure coolant aux pump is functional, and wire up motor so that it comes on with switch (normally, only comes on when cabin heat is required) -----