You CAN get to WDW without putting one wheel on I4. SR417 will take you there for around $8.00 in tolls (may be less but the tolls did go up not to long ago)Many here prefer it to I4.
A rental is usually the most cost effective transportation. National/
Alamo is on site.
Budget is off site (or was, may have changed) not sure about the others, those were all I ever used out of Sanford.
All car rental agencies are on site a SFB and are quite literally right across the street from the baggage claim area (maybe a minute walk).
Airport is nice and small, easy to get around and in/out of.
SR417 is the nice civilized road to take to WDW and the tolls were about $5-$6 last time we used SFB.
If your feeling adventurous, just take Lake Mary Blvd out through Sanford to I-4 and then I-4 west for 40 miles (watch for the Disney exit you need)
As far as Allegiantair goes, we have used them many times and they can be a great low cost option as long as you use them correctly.
***NOTE: If you are the type that must have a certain seat, be the first onto the plane, must have your free soda/snack, pack everything (and the kitchen sink) and have no flexibility in your travel days. Then Allegiant IS NOT for you!
My advice for Allegiant is:
1) being flexible with you travel days can save you quite a bit of money. Most vacationers want to leave Friday night or Saturday morning and return Sunday evening to get the most vacation time for there buck. The mid week flight days are almost always considerably cheaper, if you have the flexibility with you vacation to travel off the peak days you will save $$$
2) pack light and very carefully, DW and I will quite often share 1 checked bag (or Just take 2 carry-ons) for a 1 week trip and our 2 small backpacks as personal items (no carry-ons). Do you really need that third (and fourth) pair of shoes? Plan to have a quiet day around the resort and do a load of laundry, this way you will only have to pack enough clothes for half your trip.
3) wear you heaviest/bulkiest items on the plane (ie shoes, jacket, hoodie, jeans etc). It's usually cool on the plane anyway and a rolled up hoodie or jacket can be used as a pillow if you have a window seat or a willing partner you can lean against.
4) the earlier you arrive at the airport, the better chance you have of getting seated together. The last minute arrivals are usually the ones trying to talk people into switching seats around so they can sit together (because they didn't purchase seats and arrived late)
5) if you really have to book seats, the ones at the back of the plane are cheaper $12/seat, the front seats will cost you $25+ per seat. Allegiant boards the plane rear to front so if you have a seat near the back, overhead bin space will still be available.
6) don't bother with priority boarding, it's a waste of money since carry-ons are charged now, overhead bin space is really not an issue anymore.
7) most Allegiant flights are 3 hours or less so we typically don't need a drink or snack. You can pick up a bottle of soda or water once you are through security to take on the plane will cost you $3-$5, we don't usually bother. If you really gotta have a drink during your flight, you can buy a can of pop or a tea/coffee or snack on-board for $2-3.
8) there is no in-flight entertainment system. Put some music/movies/books on your tablet or phone or just take a book.
