Hi James, welcome to the club!

In answer to your question regarding costs, if you do some digging you'll find sourcing an Alfa motor and box is fairly cheap, if you just want those items from a breaker, although you'd be better advised to buy a complete donor car with a V5 - that way you can get the ECU and other gubbins, plus you'll be able to get an age-related reg. plate when you register the car. Having said that, the number of donors is shrinking and prices for them are rising as they edge into "classic" territory.
The Toyota motor is a harder beast to find - it's only fitted to one Toyo/Lexus car here in the UK, and it's mated to an auto box. It's also present in the back of the Lotus Evora and Exige S, usually supercharged, and fitted with a Toyota-sourced 6-speed transmission (the EA60), which requires an adapter plate and other stuff. Finding a Lotus in a scrapyard ain't easy, and it's not cheap either - I found one a couple of years back and there wasn't much change from £8K... You could go for a 5-speed box from an MR2 Turbo but you need to machine the block to fit it, and find a suitable flywheel/clutch.
For the LB installation, you'll need a dedicated ECU - LB use one from SCS. Then there are the twin fuel tanks and pumps, and the uprated radiator, and...and...

So you can see how the costs mount, compared with using a standard Alfa motor. But, you get the power as standard, and the parts should be bombproof. You can tweak the Alfa engine to about 300bhp but it costs a load, as John says above. You pays yer money, etc...