How hard would this be? (shipping option uplift)

Might be biting of more than I can chew here...

My store is going to be UK based selling only within the UK using RM Standard Parcels with an option to upgrade to RM Special Delivery Next Day. So I have no zone based shipping. Shipping modules installed:

rmstdparcels (min order value 0 max order value £34)
rmstdparcels100 (min order value £34.01 max order value £100)
rmstdparcels250 (min order value £100.01 max order value £250)
rmstdparcels500 (min order value £250.01 max order value £500)

specialdelivery (min order value 0 max order value £500)
specialdelivery1000 (min order value £500.01 max order value £1000)
specialdelivery2500 (min order value £1000.01 max order value £2500)

What I would like is to have the lowest priced option (generally standard parcels) as displays as default in step 1, automatically added to and shown in the shopping basket then have step 1 provide the option to change the shipping method to next day.

Is this a mod not worth attempting? I have a little php experience although very rusty. If it is possible any idea what pages I would need to look at.

Thanks for any help/suggestions

Neil

Zen cart 1.3.7.1
big_royal_mail_2.21a (with a couple of home made additions)