I can't comment on gremlin as I've not used them - they have been around for a while though so I presume they do a good job.
One way to save money is to only purchase the ipac, joysticks and buttons for now. Later on add the optipac, trackball and spinner. It still costs you but you can get to play some while you are still building the cab (note - this is the number one excuse for why people never finish their cabs - "I've been having too much fun playing it !!!").
Just had a quick look at the gremlin bargain pack. Its £130 which is £50 cheaper than buying the parts separately - which would be only slightly cheaper than ultimarc. I'm not keen on the gremlin buttons and the joysticks and trackball look cheap BUT like I said I've not used gremlin myself and a comment from someone who has bought parts off them would be more useful to you. One thing I'd say is if you buy all the bits seperately you will have more choice.
Did you include the coin door in your ultimarc price comparison ? Thats definitely something you could leave till last - I did but then I spent a lot of money on an authentic 2 player coin door so best not listen to me !!!!
Depending on your plans etc you could build a temprary control panel with just bottons and sticks, get used to using it then think about where the trackball/spinner will go.
Then start thinking about light guns, guitar controllers, etc etc

Worth getting IMHO is a 2 player PS-USB adaptor and 2 playstation controllers. Means you can play 4 player games and if you're a footy fan Pro Evo is excellent on the PC.
Buks