Van Purchase for sole trader

[Elm94]

I am a young self employed carpenter and I am looking to buy a new van for business use within the current financial year 2017/18 and I would like some advise on how I should treat this within my tax return.
The approximate purchase value is £9000 + VAT . I am also looking to finance the van through a standard HP agreement. I am likely to need a deposit which would be funded through part exchange of my current van which was a cash purchase in May2016 for approx. £3000. This purchase was not included in my tax return for 2016/17. In addition to the part exchange of my existing van I may need to enhance a deposit value through a cash payment.
A friend has mentioned using the Annual Investment Allowance as I have no other business assets, but I’m not sure how I represent this based upon the information above. Do I need to take into account the value of the part exchange and cash payment or do I show the purchase price Including VAT. Also would there be any benefit in making an amendment to my prior years tax return to show my original van purchase?
Furthermore can I deduct the interest on the HP agreement from my taxable profits over the period of the agreement?
I am working on getting an accountant but in the mean time I would be grateful for some advise.
Many thanks

[pawncob]

AIA will be claimed on the total cost of the van, as per the invoice, regardless of how that is made up. You could revisit last year's return, but it probably isn't worth it.
Yes you can claim the interest element of the HP payments.

[AdamS93]

Are you accounting under the accruals basis or cash basis - the answers will be very different.

[Elm94]

Thanks for the responses so far.

I'm accounting on a cash basis.

[bd6759]

No need to worry about capital allowances with cash basis,

The HP payments are treated as an expense.