A front hub motor would mean you wouldn't have to face the situation where the freehub mechanism fails on the hub motor and is difficult to replace. You would also have longer spokes on the rear wheel for more flexing and comfort which becomes more important on a 20" wheeled bike. Also ease of puncture repairs if the hub motor is on the front. Tyre wear will also be more even and you will have two wheel drive which can be beneficial on looser surfaces. If you go for a throttle only ebike kit then you can have all ebike components attached to the forks and handlebar assembly etc with use of a front rack that turns with steering so you have a very simple cable layout. It's legal to do so according to this;
www.pedelecs.co.uk
Lots of positives to having a front hub motor as well as a few negatives. Overall I feel its the better option. Ebike kits with 20" front hub motors tend to be cheaper than those with freehubs. So it may not cost more to have to buy an additional rear wheel with a freehub. You also end up with a complete spare wheelset both front and back which might be easier to sell for a better price. Also because most of the riders weight is on the rear wheel not front, typically front hub motors are more reliable, bearings last longer. Of course this depends on the design but some of the manufacturers use the same bearings on both their front and rear hub motors where as typically on a normal bicycle the front hub bearings are smaller than the rear hub bearings I think as a general rule.
Personally I would look for a folding bike secondhand which already has a freehub some can go quite cheap. The Halford's Carrera Intercity is a brilliant folding bike that can go quite cheap secondhand. Also if you have a front hub motor you can go for a bike with hub gears like the Carrera Transit which have a brilliantly reliable 3 speed Shimano Nexus and a front hub motor will take even more wear off it to extend its lifespan possibly 3x. I've also seen a Go Outdoors Compass model go quite cheap on occasion and that is a brilliant freehub based folding bike that sold for around £300 new that you see go sub £100 occasionally. I can't remember the model name but this is it with Claris based gearing. Always worth looking out for cheap Dahon or Tern models too although the ones I see often cheap are the low end entry level models with basic freewheel drivetrains despite their original price of close to £500. You have to find them with some serious discounting secondhand for them to be good value I feel as they are often twice the price they should be from new.
The important thing about going for a front hub motor is you want a folding bike with front steel forks that way you shouldn't have to bother with a torque arm which makes changing front punctures much easier. I would say as a general rule most have front steel forks but occasionally you get aluminium front forks which is where often many Dahon and Tern models are unusable unless you use a torque arm which personally I would avoid.