Should business take 100% depreciation if planning to trade vehicle in next year? What if it's financed?

Financing has nothing to do with it  and NO... you should not write it off all at once even if you can if you will not be keeping the truck for the full 5 years of the class life ... if you do you will only have to recapture the deduction  in the year of sale.  

this is really a question of tax brackets 2018 vs 2019 and time value of money.   a change in tax laws effective for 2018-2025 eliminated tax deferred exchanges for most property other than real estate 

if you fully depreciate it in 2018 you will get the $49K deduction.  whether its worthwhile or not is the question 

in 2019 when you trade it in, you will be deemed to have sold it for fair market value (which is usually the trade in value) resulting in ordinary income of that amount.   In 2019 you probably can fully write off the $54K cost of the new truck.  so say the trade in value is $36K.  so net income will go down a net of $18K

you can run scenarios using TT's taxcaster website

generally if your going to be in a high tax bracket both years,  time value of money would say take the maximum deduction each year.   but if the full 2018 deduction is going to put you in a low tax bracket while being in a high tax bracket in 2019 and subsequent , then don't   with section 179, you can actually  manipulate the depreciation  you take each year you acquire a business vehicle.  .   

another thing to consider - self employment tax if this business reported on schedule C.    the full  $49K in 2018 could save a maximum of about $7K in self employment tax

in my example over the 2 years you write offs for the trucks would net $67K.  not lets say you depreciate them not taking bonus (168k) depreciation.    depreciation/loss on sale for the two years would be about $23K(pure quesstimate) the $44K  difference would be the remaining undepreciated cost of the 2019 truck which would be spread out 2020-2024 unless disposed of beforehand.  

if the truck is the only depreciable asset acquired in 2019 and purchased in the 4th quarter - no bonus  - no 179, it would be depreciated under the mid-quarter convention and depreciation would only be 5% of cost.