The Lexicon is a great unit..... The G-Force is better and that's reflected in the price difference.
You need a unit that has dual delays, the TC Electronic G-Force and the G-Major both have dual delays, as does the Lexicon. the benefit of the G-Force over the cheaper G-Major is you can move the effects around and put them in series or parallel, where the G-Major is pretty fixed. I'm not sure about the Lexicon.
The Lexicon MPX1 is a good unit, but I think it lacks the dynamic delay of the TC 2290, which both the G-Force and G-Major has... Basically the delays in the 2 TC units are better than the Lexicon MPX1 delays. TC are pretty much industry standard when it comes to delays....... So much so that Roger Waters had a whole rack of TC Electronic D-Two's for the wall tour.... Check this out at 4:30 into the video https://www.youtube.com/watch?v=J_jDBO2ugdE
David Gilmour likes his vintage effects, that's why he still uses a Lexicon PCM70 in his rig, but he only uses that Lexicon for the circular delays at the beginning of the songs Time and Shine on You Crazy Diamond, the rest of the time he relies on the TC 2290.... The TC's do it just as good if not better now
For the money if you're on a budget and want the best delays possible..... I'd go for a TC Electronic G-Major, or G-Major 2. If you find one cheap enough get a G-Force.There is no discernible difference between the quality of the delays on the G-Force or the G-Majors.... The price difference between the 2 units is purely the flexibility in programming. Also some of the modulation effects on the G-Force are noticeably better than the G-Major.
Regarding a line mixer... no you shouldn't need one in my mind. I've seen huge guitar rack based rigs with no line mixer.