Well when it comes to normal BP yes the overall cost of the enclosure will be more then the BP, now if you have some morphs trust me the enclosure cost is nothing.

The Johnson or the Ranco will do, just keep in mind that they are not proportional and come on and off which make the temp fluctuates a little, and the more expensive ones that are proportional keep the temps constant.

Many people use Ranco and Johnson and it will be fine for your BP and if you add more BP you will be able to control the Heat Tape of other enclosure with the same thermostat.

For the heat tape I wood order 1 foot of 11 inches heat tape oh and your Tub not sure if you already bought it does not need to be that high an adult can feet in a tub no higher then 6 inches matter of fact the rest would a waste that could make it harder to control.

You can keep a BP in a tub with the following measurements

Baby – 15 Quarts (17"L x 11 1/8"W x 6 3/8"H)\
Sub Adult – 32 Quarts (23 1/2"L x 16 1/4"W x 6 3/8"H)
Adult – 41 Quarts (34 3/4"L x 16 1/2"W x 6"H)

Keep in mind that an enclosure that is too big will stress your BP

With one enclosure you will need one piece of heat tape and 2 clips and one cord you can ask to have it wire I believe at no extra charge.