I have a few of Alan's basses: http://www.acguitars.co.uk/acg_admin/wordpress/portfolio/0120-custom-j-type-fretless-4-string/ ; http://www.acguitars.co.uk/acg_admin/wordpress/portfolio/0178-recurve-4/ ; http://www.acguitars.co.uk/acg_admin/wordpress/portfolio/g025-finn-4/ ; http://www.acguitars.co.uk/acg_admin/wordpress/portfolio/g019-j-type-fretless-4/
Starting with the easy bits, I really like the flat board and asymmetric neck although I will 'fess up and say my custom fretless has a radiussed board. All of mine are under 9lbs weight and balance really well on the lap or on the strap, however if you play sitting down a lot I would probably suggest that the Finn body shape might not be the best option due to the lower horn being quite small. Personally I prefer the double cut body styles and the custom headstock shape.
I'm also a big fan of the filter preamps; I know they are not to everyones taste but are worth checking out before you make a decision. You can dial up all sorts of tones from double bass to a really agressive bark. The passive basses with the 4-way switch are also great if you prefer that arrangement, different options to VVT or VBT and I think the new ACG 2-band EQ will combine the 4-way switching option with active/passive switching with a 2-band EQ in active mode as well as still having a tone control in passive mode.
If there is any way you can get to the MBB it is worth the effort as there will be a lot of ACGs to try if the past few years are anything to go by. There are also a few on here for sale and if you get a good deal on a preloved one you get to have an extendede trial and are unlikely to lose much f anything if you move it on.
I have had a go on the Border Reiver 4-string travel bass and I think it's a great bass; so light and well balanced. I'm not aware that Alan charges more for lefties, but Dread Bass on this forum is similarly afflicted and could not doubt put you straight on this point (or just ask Alan who is really helpful). Not to mention really patient with me as I'm sure I must ask him loads of daft questions.
For what Alan charges for hand made basses you will be getting an exceptional instrument for considerably less dosh than a lot of other luthiers charge. Naturally my opinion may be a tiny bit clouded as you might imagine