IRS/OIC "flexability" ????

What is the name of your state? IL

I owe a ton of money in back income taxes, impossible to pay so I'm considering an OIC. the thing is I may need to buy a car,
an econo-car, out of necessity. I can make the payments now and for the life of the loan but after some rough number crunching it looks like my transportation expenses will exceed the IRS OIC allowance by $100-150.

Q: will the IRS actually be flexible and allow me my over the limit transportation costs (which are reasonable) or will they say I can only have what the OIC formula says? what might the IRS say?

also, this book I'm reading about the IRS/OIC says that I should "negotiate" for an extra $100-200 more per month for me rather than paid to the IRS and I should justify it to the IRS by saying I will be happier, more productive and satisfied with the IRS if they allow it.
Q: so, how to I communicate the request for less of a payment to the IRS? in a letter? the OIC literature seems pretty harsh and non-flexible to me, so I wonder; might the IRS actually agree to less of a payment for such reasons as the book says?

Q: is it true that after you submit an OIC and if the IRS thinks that they can get more out of you years later that they will reject your OIC? and that the OIC extends the collection period, i.e. the IRS could get and extra 10 years to collect?