Which step up converter to choose for a fixed Vout independent from Vin

Hi there,
for my project I require a step up converter to power a dc motor powered 
vacuum pump to run at 6.5 volts (this gives the best ratio of 
performance and noise level). The project will be powered by a single 
cell LiPo battery. If I understand the datasheets (e.g. TI lmr62014) of 
some of the lower cost step up converters correctly the Vout will be 
depending on Vin, so Vout will be higher when the LiPo cell is at 4.2 
than at around 3.7V. Is this correct and are there alternatives that 
keep Vout stable even if the LiPo voltage drops?
Also, can I use a step-up converter's enable/shutdown pin to switch on / 
off my DC motor or do I need to switch load with a separate transistor / 
fet after the converter is started up?

Thanks for your help!
Jens