Hi Michelle , I am going to try to get to that meeting myself . (I live in bali)
The Klx 150 is a small dual sport . If it was in The USA etc , you would market the bike for 16 year olds, as it has low power (but enough for here) and it is much smaller than the KLX 250 version..., which the 250cc would have been a THE far better bike . Even a 200cc Honda Tiger (road bike) would have been more comfy than a klx 150
I am 6'1" and 85kg and for me a klx 150 feels like a little kids toy with far too soft and spongy suspension that dips so much when i brake it nearly throws me over the handlebars..., but if your a lightweight you may have better luck.
If its a 2016 model though it should be very reliable though

If it is a genuine 2016 bike the bikes number plate should expire in 2012 (plates last 5 years here before you have to get new ones) the month and year expiry are stamped on the plates.
If you are 5'5" then the bike should be very comfortable for you (except for the seat) to ride and handle .
You are the first person I have heard of who has pre-organised a bike online. The price does sound high and insurance is a myth - ie there is no insurance for a rental bike , only finance insurance for an owner to cover loan defaults to the bank - there is no insurance for a rental bike..., but I guess you dont need to haggle further .
Real story is this - you crash it , or it gets stolen - then you pay in full , and good luck arguing with angry locals and the cops who will back them and not you if things go bad as their bogus insurance wont hold up.
my advice is cover your ass - always use the steering lock when you park it - or better use a disc lock as well - better to be safe than sorry.
But you did get the price online and not on the street , and you are taking the bike off the island which if it breaks down is a real nightmare for and bike owner to retrieve the broken down bike - as they themselves will need to take many days from their work/ lives etc off to cross islands on ferries etc with a truck (all at great expense) just to sort the bike out if it runs into issues..., so thats the reason for the higher price.
Allow to days to get to sumbawa - it can be made in a day is you leave at dawn , but you wont get there till midnight and it'll be a stressful and tiring day .., better to break up the ride with an overnight stop in Lombok
Usually rental bikes are negotiated on the street here in Bali - you can wing it to other islands behind the rental owners back but just make sure you take full responsibility if you do upon yourself to either fix, or get the bike back safely at whatever expense to yourself.
Alternatively you can tell them your intentions and they will surely nego a higher price (as you have already done) but also make sure you get everything in writing regarding the rental and more importantly the written permission from the registered owner that they permit you to take it off the island.
Police checkpoints at ferry crossings will want to check this before you board the ferries , and will want to see your licence , passport, and the bikes original registration papers ..,
tip - keep RP50.000 (about US3.50) ready folded in a top pocket , and the very second you feel a police speech is about to begin (telling you that you have broken an imaginary rule etc ) just save yourself the lengthy sermon the hassles and very discretely slip him the 50,000 rupiah note , smile and say this is for you children's school (or similar) and your instantly on your way.
also bring lots of rope, those skinny bikes have no carrying capacity , and likely not even a tiny rack , so somehow you will need to lash your bag etc down onto the seat with the rope somehow.
Be careful in Bali on the bike , as it is a madhouse of crazy death defying traffic, but very relaxed once off the island to the east
cheers